Abstract: The oncogenes (v-onc genes) of rapidly transforming retroviruses have homologs (c-onc genes) in the genomes of normal cells. In this study, we characterized and quantitated transcription from four c-onc genes, c-myb, c-myc, c-erb, and c-src, in a variety of chicken cells and tissues. Electrophoretic analysis of polyadenylated RNA, followed by transfer to nitrocellulose and hybridization to cloned onc probes showed that c-myb, c-myc, and c-src each give rise to a single mature transcript, whereas c-erb gives rise to multiple transcripts (B. Vennstrom and J. M. Bishop, Cell, in press) which vary in abundance among different cells and tissues. Transcription from c-myb, c-myc, c-erb, and c-src was quantitated by a "dot-blot" hybridization assay. We found that c-myc, c-erb, and c-src transcription could be detected in nearly all cells and tissues examined, whereas c-myb transcription was detected only in some hemopoietic cells; these cells, however, belong to several different lineages. Thus, in no case was expression of a c-onc gene restricted to a single cell lineage. There appeared to be a correlation between levels of c-myb expression and hemopoietic activity of the tissues and cells examined, which suggests that c-myb may be expressed primarily in immature hemopoietic cells. An examination of c-onc RNA levels in target cells and tissues for viruses carrying the corresponding v-onc genes revealed no obvious correlation, direct or inverse, between susceptibility to transformation by a given v-onc gene and expression of the homologous c-onc gene.

Abstract: Eleven cases of a small cell ovarian cancer associated with hypercalcemia that was reversed by removal of the tumor are reported and compared with three similar cases in the literature. The 14 cases account for 50% of recorded cases of ovarian-cancer-related hypercalcemia. The 14 tumors occurred in women between the age of 13 and 35, with an average of 22 years and, with two exceptions, behaved clinically in an aggressive fashion. Light and electron microscopic study confirmed the epithelial nature of the neoplasms but failed to disclose their specific subtype or origin.

Abstract: In human cells treated with interferons, there is an increase in the amount of HLA-A,B,C and beta 2-microglobulin exposed on the cell surface. We have used a cloned HLA-A,B,C cDNA probe to demonstrate by molecular hybridization that this effect of interferon is preceded by a large increase in the amount of HLA mRNA in the cell. This effect was found in five different human cell lines, with purified leukocyte and fibroblast interferons. The increase in HLA mRNA is comparable in its kinetics and dose-response to the induction of (2'-5') oligo(A) synthetase mRNA by interferons. Therefore, interferons seem to activate at least two cellular genes which have different biochemical functions.

Abstract: Epidermal growth factor (EGF) may be important in regulating the proliferation of mammary epithelial cells. In the present study, we examined EGF binding and effect on growth in nine human mammary cell lines. The T-47D, MCF-7, SK-Br-3, AIAb 496, BT-20, and BT-474 tumor cell lines and a cell line (HBL-100) derived from milk exhibited EGF binding; both high (Ka 10(10) M-1)- and low (Ka 10(9) M-1)-affinity sites were detected. The total number of EGF receptors per cell of different cell lines varied from 1.6 X 10(3) sites/cell (for AIAb 496) to 1.5 X 10(6) sites/cell (for BT-20). The two floating cell lines, DU4475 and Lev III, had no detectable EGF binding. Effect of EGF on growth was studied by monitoring cell number and the incorporation of [3H]thymidine into DNA of cells maintained in Dulbecco's modified Eagle's medium supplemented with 0.1% fetal bovine serum. Using these procedures, only T-47D cells were stimulated by EGF at low concentrations (0.1 to 1 ng/ml). At concentrations higher than 10 ng/ml, EGF was inhibitory to varying degrees in most cell lines that contained EGF receptors. The growth of the two floating cell lines that had no detectable EGF binding was unaffected by EGF. Our results show that EGF receptors are not present in all human breast cancer cell lines. There is no apparent correlation between EGF binding and its mitogenic activity in cell lines with EGF receptors. EGF may have biological roles in human breast cancer other than growth regulation.

Abstract: The P3J-HR-1 (HR-1) line contains unique Epstein–Barr viruses (EBVs) which fail to immortalize lymphocytes or to induce lymphomas1–4. Virus from Jijoye, the parent of the HR-1 clone, retains lymphocyte-immortalizing ability5,6. Because the HR-1 virus population is a mixture, it has not been possible to identify conclusively the genomic alterations responsible for the nontransforming phenotype7. Submolar fragments produced by restriction enzyme digestion and the variety of patterns found after partial denaturation are evidence for heterogeneity in HR-1 virion DNA8,9. To determine whether every cell carried this virus mixture, or whether the line consisted of cells carrying different variants, we separated the mixture by cell subcloning into new variants, which are found to be distinct from parental HR-1 virus both biologically and in genome structure.

Abstract: A number of cell lines have been derived from bone marrow cultures in the presence of WEHI-3 conditioned media (CM) that continue to require WEHI-3 CM for growth in vitro. Because the WEHI-3 cell line has been shown to constitutively produce a lymphokine (IL 3) that induces the expression of 20 alpha-hydroxysteroid dehydrogenase (20 alpha SDH) in cultures of splenic lymphocytes from athymic mice, we examined whether these cell lines were dependent upon IL 3 for growth. The results demonstrate that the factor required for growth of these cell lines copurifies with IL 3 activity on G-100, DEAE cellulose, and CM cellulose column chromatography as well as in preparative isoelectric focusing and on hydrophobic supports in reverse phase high pressure liquid chromatography. The biologic activity of peak fractions in each case was similar in both types of assays. These results strongly suggest that the WEHI-3 CM-dependent cell lines are dependent on IL 3 for growth in vitro. All the cell lines have readily detectable levels of 20 alpha SDH but have differing cell surface phenotypes. The C3HSFFV line is devoid of conventional lymphoid cell surface markers with the exception of Lyt-1, whereas the FDC-P1 expresses Thy-1, Ly-5, and H-11. Other cell lines have intermediate phenotypes.

Abstract: Several lines of evidence suggest tha Ca2+ ions control cell proliferation: Ca2+ entry into cytoplasm acts as a general mitogen; serum and serum-replacements induce Ca2+ influx; the Ca2+ concentrations in growth media required to support the proliferation of normal cells are much higher than those required for cancer cells; serum and growth factors reduce the Ca2+ requirements of normal cells; tumour promoters alter Ca2+ fluxes via a mechanism used principally by growth factors. Minor supporting evidence includes the effects of various drugs and viruses, and the behaviour of tumour cell mitochondria and intercellular junctions. It is still not possible to decide exactly where and when inside cells the critical effect of Ca2+ on proliferation occurs, but we discuss at length the practical problems of understanding Ca2+ movements in tissue-culture cells. Carried to its logical conclusion, present evidence suggests that an overridden or bypassed Ca2+ control process may be the key, common determinant of unrestrained proliferation in cancer cells.

Abstract: A rat monoclonal antibody against the murine transferrin receptor has been identified. The receptor is a 95,000 molecular weight species that exists in the cell membrane as a disulphide-bonded dimer. Whereas 29 of 29 murine hematopoietic tumor cell lines express detectable numbers of transferrin receptors, less than 1% of adult thymocytes or spleen cells and only 5% of bone marrow cells are positive. However, fetal liver and neonatal spleen contain substantial numbers of transferrin receptor-positive cells. Induction of Friend cells in vitro with dimethyl-sulphoxide leads to an overall increase in the expression of transferrin receptors on the cell surface. The anti-transferrin receptor antibody we have obtained partially blocks iron uptake from 59Fe-transferrin by a variety of murine cell lines and inhibits the growth of a murine myeloma cell line in vitro.

Abstract: IgG-PFC was induced in Epstein-Barr virus-transformed B lymphoblastoid cell lines (LCL) by the addition of allogeneic T cells. T cells involved in the induction of IgG-PFC were shown to belong to the Leu 3a+/2a- T cell subset. Furthermore, partially purified soluble factors obtained from the culture supernatant of PPD-stimulated pleural T cells or PWM-stimulated tonsillar mononuclear cells was shown to induce IgG-PFC in LCL across the major histocompatibility complex barrier. The induction of IgG-PFC was observed only in surface IgG-positive LCL cell populations and was not accompanied by the increase in the number of LCL cells. The factors with such a TRF-like activity were found in two fractions corresponding to the m.w. range of 18,000 to 25,000 (22K fraction) and 28,000 to 38,000 (36K fraction) by gel filtration. Isoelectric focusing of these fractions revealed that TRF-like activity of both 22K and 36K fractions distributed in the pI range of 5.0 to 6.0, and both fractions were found to be devoid of TCGF activity. These results appear to indicate that the factors act on the B cells in terminal stages to trigger final differentiation to Ig-producing cells.

Abstract: Studies of the reaction of antibody A2B5 and tetanus toxin with pancreatic islet cells, islet cell tumors, and other human amine precursor uptake and decarboxylation (APUD) tumors are described. By indirect immunofluorescence, antibody A2B5 and tetanus toxin were shown to specifically bind to the plasma membrane of human, rat, chicken, and mouse islet cells. The binding of antibody A2B5 to the cell surface of living islet cells has allowed isolation of these cells from a suspension of pancreatic cells by using a fluorescence-activated cell sorter. In studies designed to determine whether tetanus toxin and antibody A2B5 bound to the same surface antigen, A2B5 and tetanus toxin did not compete for binding to normal islet cells, a human islet cell tumor, or a rat islet cell tumor. In addition to binding to islet cell tumors, antibody A2B5 reacts with frozen sections, isolated cells, and cell lines of neural, neural crest, and APUD origin.

Abstract: In contrast to the many reports of IL-2 dependent proliferating, helper or killer cell clones1, there is only a single report of an IL-2 dependent suppressor cell clone2, in the mouse. However by the same cloning procedures used to generate human helper cells3, suppressor cell clones to influenza virus could not be generated, and so another strategy was used. Jerne's network hypothesis4 proposes that immune regulation results from lymphoid cell receptors recognizing determinants on other lymphoid cell receptors. If this is the case it should be possible to generate regulatory T cell clones against other T cells and we report here the generation of an autologous suppressor cell clone which recognizes and inhibits the function of a human helper T cell clone. Such an autologous suppressor cell clone provides a new approach to understanding the pathways and molecular events involved in immune regulation. Mutual stimulation of the suppressor cell clone and the target helper cell clone in the absence of back stimulation, provides direct experimental evidence for the existence of interactions between T cell receptors, and thus suggests that the specificity of the suppressor cell clone is for the antigen receptor of the helper cell.

Abstract: We have analyzed the effect of estradiol and of two classes of antiestrogens on the morphology of the MCF7 human breast cancer cell line by scanning and transmission electron microscopy. Estradiol progressively increased the number and the length of microvilli at the cell surface. The density of the microvilli network increased between 2 and 11 days of estrogen treatment, while the cells became more globular and less tightly attached to the surface of the dish. Estradiol also progressively transformed cells into secretory cells containing, at Day 2, large, clear mitochondria and, at Day 4, rough endoplasmic reticulum and Golgi complexes. At Day 6, secretory granules (diameter, 0.2 µm), which mainly contained glycoproteins, were first observed in the cytoplasm. By Day 8, they were concentrated at the cell membrane and being liberated into the medium. Larger granules (diameter, 0.8 µm), which probably contained lipids, were observed later (Day 11). Cell cultures in 10% fetal calf serum not treated by charcoal contained secretory granules. The modifications were induced by physiological concentrations of estradiol but not 5α-dihydrotestosterone. Progesterone (10 nm for 8 days) completely inhibited the effect of estradiol on the microvilli and secretory activity. Tamoxifen or hydroxytamoxifen did not induce secretory activity but did alter the cell morphology compared to control cells. The effects of estradiol were observed in other estrogen receptor-positive breast cancer cell lines (ZR 75-1, T 47 D) but not in an estrogen receptornegative cell line (BT 20). This morphological evidence that estrogens modify the cell surface of breast cancer cells in culture and transform them into “secretory cells” complements evidence that they induce the release of a glycoprotein with a molecular weight of ≃50,000 into the culture medium (Cell, 20: 352–362, 1980). (The molecular weight was found first to be 46,000. It seems to be closer to 52,000 in a 10% polyacrylamide gel and by using the NEN-labeled proteins as molecular weight markers.)

Abstract: Erythrocytes infected with Plasmodium falciparum trophozoites and schizonts are not seen in the peripheral circulation because they attach to venular endothelium via knoblike structures on the infected erythrocyte membrane. We have recently shown that erythrocytes containing P. falciparum trophozoites and schizonts likewise attach to cultured human venous endothelial cells via knobs. In search of a more practical target cell for large scale binding studies designed to characterize and isolate the knob ligand, we tested various normal cells and continuous cell lines for their ability to bind P. falciparum-infected erythrocytes. Of the 18 cell types tested, binding of infected erythrocytes was observed to a human amelanotic melanoma cell line and amnion epithelial cells as well as to human aortic and umbilical vein endothelial cells. 96-100% of amelanotic melanoma cells bound 17±4 (±1 SEM) infected erythrocytes per positive cell, whereas fewer endothelial cells (4-59%) and amnion epithelial cells (8-19%) were capable of binding 12±5 and 4±1 infected erythrocytes per positive cell, respectively. Further studies designed to compare the mechanism of binding to the amelanotic melanoma cell line and endothelial cells showed the following results. First, that adhesion of infected erythrocytes to these two cell types was parasite stage-specific in that only erythrocytes containing late ring forms, trophozoites, and schizonts bound. Erythrocytes containing early ring forms, which do not attach to venular endothelium in vivo, did not bind to either cell type. Second, erythrocytes infected with trophozoites and schizonts of P. vivax or a knobless strain of P. falciparum, both of which continue to circulate in vivo, did not bind to either target cell type. Third, transmission electron microscopy showed that infected erythrocytes attached to the amelanotic melanoma cells via knobs. We conclude that cultured human endothelial cells and an amelanotic melanoma cell line share common determinants on their surface and that the mechanism of binding to these two different cell types is similar. The amelanotic melanoma cell line offers a useful substitute for endothelial cells in binding studies requiring large numbers of target cells.

Abstract: Human breast epithelial cells cultured from milk have been transformed with SV40. Indirect immunofluorescence tests using monoclonal antibodies show that cells from clones grown in soft agar have SV40 large T-antigen in their nuclei and epithelia-specific tonofilament antigens on their intermediate filaments. In primary cultures of milk epithelial cells, the tonofilaments form a characteristic delicate basketwork throughout the cytoplasm, but in the SV40-transformed epithelial cell strains, the filament network is grossly distorted. Insulin, hydrocortisone, and serum stimulate the growth of the cell strains. At passages 8 to 11, the cell strains become quiescent and usually die. One cell strain survived this crisis period and gave rise to the fR series of cell lines. Most cell lines have a cuboidal morphology and react with a monoclonal antibody that recognizes a differentiation antigen on the membranes of breast epithelia. Line fR2 expressed the highest level of this antigen whereas fR5, the only fR line isolated with fusiform morphology, had relatively little. The in vitro-transformed lines may be related to the two dominant epithelial cell types seen in primary milk cultures and could be useful for studying the relationship between transformation and differentiation in human mammary epithelial cells.

Abstract: SummaryInvestigations into plant intercellular communication were initiated through an examination of plasmodesmata and cell-to-cell passage of molecular probes in the staminal hairs ofSetcreasea purpurea. Plasmodesmata connecting staminal hair cells of small buds are filled with an electron-opaque homogenous material. To examine the permeation selectivity of plasmodesmata, molecular probes made up of fluorescein isothiocyanate (FITC) complexed with amino acids and peptides were injected into the staminal hair cells and the spread of these fluorescent molecules through the symplast, was monitored. Molecules composed of FITC complexed to single amino acids with polar and aliphatic R groups travel rapidly, while those which include peptides travel slowly. Dye molecules composed of an amino acid with an aromatic side group do not pass from cell to cell at all. It is hypothesized that the material occluding the plasmodesmata constitutes the diffusion barrier, by presenting a hydrophilic environment which allows passage of molecules with maximum molecular weights of 700–800 daltons, but which retains those with aromatic side groups.

Abstract: The activation of B lymphocyte subpopulations by anti-immunoglobulin and by LPS has been examined. All resting B cells were stimulated to grow larger (i.e. to go from G0 phase to mid G1 phase of the cell cycle) by the continuous presence of anti-mu antibodies. A subpopulation oif these B cells, 30-50% in normal mouse strains, entered S phase in response to large doses of anti-mu. This subpopulation, probably Lyb5+, was completely absent in mice with the xid-determined immune defect. Another, apparently distinct subpopulation, comprising about 25% of the cells, and probably present in xid mice, was sensitive to a proliferative signal delivered by LPS, if the cells had first been cultured for 24 h in the presence of a dose of anti-mu that was sufficient to cause cell enlargement. The fraction of B cells responding to LPS in this way was significantly larger than the fraction responding to LPS alone, suggesting that anti-mu is superior to LPS at inducing the G0 to G1 transition. Based on these results we propose a model of the control of B cell growth and proliferation. Anti-Ig antibodies, or epitopes on conventional antigens, combine with and cross-link B cell receptors, causing the cells to enter G1 and to develop sensitivity to late G1 stimuli, which determine whether they will then enter S phase. These stimuli are provided either by a high dose of anti-mu or by LPS. These agents may work directly or may stimulate other cells to produce B cell Growth Factor (BCGF) and/or related regulatory molecules which may be the actual late G1 stimuli. Distinct B cell types are sensitive to distinct mechanisms for control of proliferation. A new monoclonal antibody, 14G8, which recognizes only a fraction of B cells (30% in normal mice and about 65% in xid mice), was used to separate B cell subpopulations based on the presence or absence of the cell surface antigen recognized by this antibody. The results suggest that 14G8 expression is negatively correlated with Lyb5 expression, although not absolutely. Indeed 14G8+ B cells respond quite well to anti-mu (32% the cells enter S phase). Since Lyb5- B cells are believed not to proliferate in response to anti-mu, this would suggest that a sizeable fraction of the 14G8+ B cells are also Lyb5+. The 14G8+ and 14G8- B cell subpopulations were found to be functionally distinct in that the former responded very well to LPS, whereas the latter responded very poorly. Models of B cell development based on expression of these membrane antigens are presented.

Abstract: Leukemic cells from 70% of patients with Ia+CALLA+ non-T cell acute lymphoblastic leukemia (ALL) express an antigen (B1) found on all normal B lymphocytes. In this study, ALL cells that do not express the B1 antigen were studied in an attempt to further elucidate the cellular lineage of these tumors. Non-T cell ALL lines and tumor cells isolated from patients with non-T cell ALL that are Ia + CALLA + B1- were studied in vitro with a variety of agents known to promote cellular differentiation. Phorbol diester (TPA) or phytohemagglutinin conditioned leukocyte culture media were capable of inducing the expression of B1 on all four non-T cell ALL lines tested. In contrast, B1 could not be induced under the identical conditions on a promyelocytic leukemia line or a T cell lymphoblastic leukemia line. With the induction of B1 on non-T cell ALL lines, cytoplasmic mu-heavy chain (c mu) became undetectable, whereas the expression of CALLA and Ia were unchanged. The expression of B1 was accompanied by a decrease of cellular proliferation and DNA synthesis, but not significant morphologic changes were noted. In addition, no other B or T cell antigens were detected. The cellular origin of non-T cell ALL was further investigated using tumor cells isolated from leukemic patients. Tumor cells from eight patients with Ia + CALLA + B1-c mu- ALL could be induced in vitro with TPA to express both B1 and c mu. In contrast, cells from five patients with Ia + CALLA-B1-c mu- non-T cell ALL could not be induced with TPA to express CALLA, B1 or c mu. These studies suggest that the non-T cell ALL are heterogeneous and represent a spectrum of early B cell differentiation including the pre- pre-B cell (Ia + CALLA + B1-c mu-), the intermediate pre-B cell (Ia + CALLA +B1 + c mu-), and finally the "true" pre-B cell (Ia + CALLA + B1 + c mu+). The cellular origin of the remaining Ia + CALLA-B1-c mu- form of non-T cell ALL (20%) is still unknown.

Abstract: The water soluble beta-adrenergic ligand [3H]CGP-12177 was used to measure the cell surface receptors in intact cells. In two cell lines, C6 glioma and WEHI 7 lymphoma cells, -50% of the cell surface receptors disappear within minutes of incubation of the cells with isoproterenol. The receptors can still be detected in homogenates and reappear on the cell surface when cells are washed and reincubated at 37 degrees C. The data agree with a disappearance of the receptors from the cell surface by an agonist-mediated endocytosis.

Abstract: Two cell populations were isolated from calvaria of chick embryos: PF cells were liberated by collagenase treatment from the periosteum, OB cells from the periosteum-free calvarium Both populations were cultured in plastic culture dishes After 6 d of culture, monolayers of each cell type either were scraped off the culture dishes, transplanted on the chorio-allantoic membrane of 7-d-old quail eggs, and cultured there for 6 d, or were used for biochemical experiments OB transplants proved capable of producing calcified bone matrix, whereas PF transplants formed only fibrous tissue Biochemically, OB cells showed high cAMP production in the presence of parathyroid hormone (PTH), whereas cAMP production was not stimulated in PF cultures Lactate production was stimulated by PTH in both populations although somewhat differently Citrate decarboxylation was high in OB cells and was inhibited by PTH but was low in PF cells, where it was stimulated by the same hormone The differences in hormonal response between the two cell types made it possible to conclude that PF cultures are relatively free of OB cells The PF contamination in OB cultures was more difficult to assess The experiments described in this report show that the OB population contains osteoblasts or osteoblastlike cells which are, under favorable circumstances, capable of bone formation

Abstract: We have isolated continuously growing cell lines derived from mouse bone marrow stroma. These cell lines were independently obtained, and though they showed morphologies ranging from the epithelioid to the fibroblastoid patterns, they all differentiated into adipocytes. Subclones obtained from two cell lines had a very high frequency (90-100%) of differentiation into adipocytes after two or three weeks of arrested growth. Though extensive accumulation of lipid often mechanically impaired mitosis, the cells committed to adipocytes did not suffer an irreversible loss of proliferative capacity. Adipogenesis was obtained in conditions similar to those required for fat cell formation in long-term bone marrow culture. The cell lines were found to be insensitive to insulin as a signal of adipocyte differentiation. The ultrastructural characteristics of the preadipocytes and fat cells are also similar to those of the fat cells developing in long-term bone marrow culture. As such, these cell lines should prove useful for analysing cell/cell interactions in haemopoiesis.

Abstract: Phorbol esters are potent inducers of macrophage-like differentiation in the HL-60 promyelocytic leukemia cell line The sequence of events by which they bring about this transition is poorly understood However, it is known that phorbol esters bind to the surface membrane of HL-60 cells and to various other cells as well Our studies were directed toward determining the biologic importance of this membrane association [3H]Phorbol dibutyrate (PBu2) was specifically bound by HL-60 cells with a Kd of 23 nM and with 19 X 10(5) binding sites for [3H]PBu2 per cell There was no internalization of bound [3H]PBu2 Specific binding was fully reversible upon washing in fresh medium, and [3H]PBu2 added thereafter bound normally to its receptor Within 10 min of binding, PBu2 stimulated [14C]choline incorporation into phosphatidylcholine, with a rapid return to normal upon removal of the PBu2 Membrane-bound PBu2 progressively inhibited DNA synthesis, with 70% inhibition by 8 hr This process was interrupted if the PBu2 was removed, and little recovery of DNA synthesis occurred in previously inhibited cells Between 8 and 16 hr, PBu2 induced adherence of cells to plastic, but only in those cells in which phosphatidylcholine synthesis was stimulated, and this process was also interrupted if PBu2 was removed prior to 16 hr Similarly, nonspecific esterase, which develops after 72 hr of incubation, was induced in cells exposed to PBu2 for the initial 16 hr but not in cells exposed for 5 hr These studies demonstrate that phorbol esters exert their effects while retained at the cell surface Inhibition of cell growth and the acquisition of surface and enzymatic properties that characterize macrophages are separable events, each of which proceeds through a receptor-mediated, transmembrane process The stimulation of phosphatidylcholine synthesis appears to be a part of that process

Abstract: Hematologic stem cells pluripotent for lymphoid and myelogenous cells have not been identified directly in humans. However, several indirect approaches have been used to study this question. For example, the existence of a progenitor pluripotent for lymphoid as well as myeloid cells is suggested by the finding of both cell types in individual, presumably clonal, colonies grown in vitro from human marrow (Messner et al., 1981). The indirect method used in the work reviewed here is based upon study of hematologic neoplasms arising in females whose X-chromosome in-activation mosaicism is delineated with glucose-6-phosphate dehydrogenase (GGPD) markers. Since the G6PD locus is on the X chromosome and undergoes inactivation, only one of the two G6PD genes is active in somatic cells of females. Thus, women with the usual G6PD gene (GdB) on one X chromosome and the common variant GdA on the other X have two populations of cells, one producing A-type G6PD and the other B-type G6PD. The two enzymes can easily be distinguished from one another by starch-gel or cellulose-acetate electrophoresis. The presence of G6PD-marked cellular mo-saicism allows studies of such questions as