Single-cell transcriptome analysis of endometrial tissue

STUDY QUESTION

How can we study the full transcriptome of endometrial stromal and epithelial cells at the single-cell level?

SUMMARY ANSWER

By compiling and developing novel analytical tools for biopsy, tissue cryopreservation and disaggregation, single-cell sorting, library preparation, RNA sequencing (RNA-seq) and statistical data analysis.

WHAT IS KNOWN ALREADY

Although single-cell transcriptome analyses from various biopsied tissues have been published recently, corresponding protocols for human endometrium have not been described.

STUDY DESIGN, SIZE, DURATION

The frozen-thawed endometrial biopsies were fluorescence-activated cell sorted (FACS) to distinguish CD13-positive stromal and CD9-positive epithelial cells and single-cell transcriptome analysis performed from biopsied tissues without culturing the cells. We studied gene transcription, applying a modern and efficient RNA-seq protocol. In parallel, endometrial stromal cells were cultured and global expression profiles were compared with uncultured cells.

PARTICIPANTS/MATERIALS, SETTING, METHODS

For method validation, we used two endometrial biopsies, one from mid-secretory phase (Day 21, LH+8) and another from late-secretory phase (Day 25). The samples underwent single-cell FACS sorting, single-cell RNA-seq library preparation and Illumina sequencing.

MAIN RESULTS AND THE ROLE OF CHANCE

Here we present a complete pipeline for single-cell gene-expression studies, from clinical sampling to statistical data analysis. Tissue manipulation, starting from disaggregation and cell-type-specific labelling and ending with single-cell automated sorting, is managed within 90 min at low temperature to minimize changes in the gene expression profile. The single living stromal and epithelial cells were sorted using CD13- and CD9-specific antibodies, respectively. Of the 8622 detected genes, 2661 were more active in cultured stromal cells than in biopsy cells. In the comparison of biopsy versus cultured cells, 5603 commonly expressed genes were detected, with 241 significantly differentially expressed genes. Of these, 231 genes were up- and 10 down-regulated in cultured cells, respectively. In addition, we performed a gene ontology analysis of the differentially expressed genes and found that these genes are mainly related to cell cycle, translational processes and metabolism.

LIMITATIONS, REASONS FOR CAUTION

Although CD9-positive single epithelial cells sorting was successfully established in our laboratory, the amount of transcriptome data per individual epithelial cell was low, complicating further analysis. This step most likely failed due to the high dose of RNases that are released by the cells' natural processes, or due to rapid turnaround time or the apoptotic conditions in freezing- or single-cell solutions. Since only the cells from the late-secretory phase were subject to more focused analysis, further studies including larger sample size from the different time-points of the natural menstrual cycle are needed. The methodology also needs further optimization to examine different cell types at high quality.

WIDER IMPLICATIONS OF THE FINDINGS

The symbiosis between clinical biopsy and the sophisticated laboratory and bioinformatic protocols described here brings together clinical diagnostic needs and modern laboratory and bioinformatic solutions, enabling us to implement a precise analytical toolbox for studying the endometrial tissue even at the single-cell level.

Differentiation and molecular profiling of human embryonic stem cell-derived corneal epithelial cells

It has been suggested that the isolation of scalable populations of limbal stem cells may lead to radical changes in ocular therapy. In particular, the derivation and transplantation of corneal stem cells from these populations may result in therapies providing clinical normality of the diseased or damaged cornea. Although feasible in theory, the lack of donor material in sufficient quantity and quality currently limits such a strategy. A potential scalable source of corneal cells could be derived from pluripotent stem cells (PSCs). We developed an in vitro and serum-free corneal differentiation model which displays significant promise. Our stepwise differentiation model was designed with reference to development and gave rise to cells which displayed similarities to epithelial progenitor cells which can be specified to cells displaying a corneal epithelial phenotype. We believe our approach is novel, provides a robust model of human development and in the future, may facilitate the generation of corneal epithelial cells that are suitable for clinical use. Additionally, we demonstrate that following continued cell culture, stem cell-derived corneal epithelial cells undergo transdifferentiation and exhibit squamous metaplasia and therefore, also offer an in vitro model of disease.

In vitro and in vivo metabolism of a novel cannabinoid-1 receptor inverse agonist, taranabant, in rats and monkeys

The metabolism and excretion of taranabant (MK-0364, N-[(1S,2S)-3-(4-chlorophenyl)-2-(3-cyanophenyl)-1-methylpropyl]-2-methyl-2{[5-(trifluoromethyl)pyridine-2-yl]oxy}propanamide), a potent cannabinoid-1 receptor inverse agonist, were evaluated in rats and rhesus monkeys. Following administration of [¹⁴C]taranabant, the majority of the radioactivity was excreted within 72 h. In both rats and rhesus monkeys, taranabant was eliminated primarily via oxidative metabolism, followed by excretion of metabolites into bile. Major pathways of metabolism that were common to rats and rhesus monkeys included hydroxylation at the benzylic carbon adjacent to the cyanophenyl ring to form a biologically active circulating metabolite M1, and oxidation of one of the two geminal methyl groups of taranabant or M1 to the corresponding diastereomeric carboxylic acids. Oxidation of the cyanophenyl ring, followed by conjugation with glutathione or glucuronic acid, was a major pathway of metabolism only in the rat and was not detected in the rhesus monkey. Metabolism profiles of taranabant in liver microsomes in vitro were qualitatively similar in rats, rhesus monkeys and humans and included formation of M1 and oxidation of taranabant or M1 to the corresponding carboxylic acids via oxidation of a geminal methyl group. In human liver microsomes, metabolism of taranabant was mediated primarily by CYP3A4.

Granulocyte colony-stimulating factor (G-CSF) depresses angiogenesis in vivo and in vitro: implications for sourcing cells for vascular regeneration therapy

SUMMARY BACKGROUND

The most common source of hematopoietic progenitor cells (HPCs) for hematopoietic reconstitution comprises granulocyte colony-stimulating factor (G-CSF)-mobilized peripheral blood stem cells (PBSCs). It has been proposed that endothelial progenitor cells (EPCs) share precursors with HPCs, and that EPC release may accompany HPC mobilization to the circulation following G-CSF administration.

OBJECTIVE

To investigate EPC activity following HPC mobilization, and the direct effects of exogenous G-CSF administration on human umbilical vein endothelial cells (HUVECs) and endothelial outgrowth cells (EOCs), using in vitro and in vivo correlates of angiogenesis.

PATIENTS/METHODS

Heparinized venous blood samples were collected from healthy volunteers and from cord blood at parturition. G-CSF-mobilized samples were collected before administration, at apheresis harvest, and at follow-up. PBSCs were phenotyped by flow cytometry, and cultured in standard colony-forming unit (CFU)-EPC and EOC assays. The effect of exogenous G-CSF was investigated by addition of it to HUVECs and EOCs in standard tubule formation and aortic ring assays, and in an in vivo sponge implantation model.

RESULTS

Our data show that G-CSF mobilization of PBSCs produces a profound, reversible depression of circulating CFU-EPCs. Furthermore, G-CSF administration did not mobilize CD34+CD133- cells, which include precursors of EOCs. No EOCs were cultured from any mobilized PBSCs studied. Exogenous G-CSF inhibited CFU-EPC generation, HUVEC and EOC tubule formation, microvessel outgrowth, and implanted sponge vascularization in mice.

CONCLUSIONS

G-CSF administration depresses both endothelial cell angiogenesis and monocyte proangiogenic activity, and we suggest that any angiogenic benefit observed following implantation of cells mobilized by G-CSF may come only from a paracrine effect from HPCs.

Blood donor derived dendritic cells and cytotoxic T cells for specific fusion-gene adoptive immunotherapy

BACKGROUND AND OBJECTIVES

Therapeutic immunological reagents tailored to individual patients have been shown to be a viable treatment strategy for some forms of leukaemia. This work investigates the possibility of using blood donations as a source of leukaemia-specific immune therapeutics.

MATERIALS AND METHODS

The acute promyelocytic cell line NB4 carrying the PML-RAR alpha fusion was used as a target for cytotoxic T lymphocytes (CTL) stimulated to recognize the fusion. Stimulation of CTL was by production of dendritic cells pulsed with plasmid vectors containing polymerase chain reaction (PCR)-generated sequences of PML-RAR alpha derived from NB4 cells. PCR primer design included a Kozak consensus sequence to allow correct translation of the nucleic acid into protein. Identification of specific cytotoxicity was by both Granzyme B ELISPOT and by (51)Cr-release assays.

RESULTS

Specific CTL activity targeting NB4 cells can be generated from donor-derived peripheral blood mononuclear cells. However, individual donors appear to respond differently to the length of stimulatory sequence encoded in the vector. Use of an internal methionine in the PML gene, which also satisfies the Kozak rules, allows translation in vitro and, thus, might provide a suitable start site for stimulation using acute promyelocytic leukaemia-specific sequence.

CONCLUSION

The work presented here suggests that blood donor derived dendritic cells can be used to stimulate leukaemia-specific CTL from the same donation ex vivo. This would enable the generation of patient-specific therapeutics from major histocompatibility (MHC)-matched allogeneic donors. However, different MHC-matched donors might vary in their response depending on the length of the antigenic sequence.

Phase I/II study of vaccination with dendritic-like leukaemia cells for the immunotherapy of acute myeloid leukaemia

Twenty-two patients with acute myeloid leukaemia were recruited into a phase I/II clinical trial investigating the vaccination of patients in complete remission (CR) with autologous dendritic-like leukaemia cells (DLLC). At trial entry, leukaemia cells were harvested and tested for their ability to undergo cytokine-induced dendritic cell differentiation. Patients were then treated with intensive chemotherapy. Five patients achieved both CR and had leukaemia cells that successfully underwent differentiation and therefore proceeded to vaccination. Four escalating doses of DLLC were administered weekly by subcutaneous injection. Vaccination was generally well tolerated although one patient developed extensive eczema and an increased antinuclear factor titre possibly indicating induction of autoimmunity. Development of anti-leukaemic T-cell responses was assessed by enzyme-linked immunospot analysis of gamma-interferon secreting T lymphocytes and by human leucocyte antigen tetramer analysis for WT1-specific T cells. Increases in anti-leukaemic T-cell responses were demonstrated in four patients, but only two of the five remained in remission more than 12 months postvaccination. The study has demonstrated that generation of DLLC is feasible in only a subgroup of patients and is currently neither broadly applicable or clinically effective.

Fusion hybrids of dendritic cells and autologous myeloid blasts as a potential cellular vaccine for acute myeloid leukaemia

We assessed the potential of tumour cell/dendritic cell fusion hybrids to generate in vitro anti-leukaemic T-cell responses following co-culture with autologous remission lymphocytes in six patients with acute myeloid leukaemia (AML). Comparison was made to anti-leukaemic responses induced by mature dendritic cells (mDC) co-cultured with autologous, irradiated myeloid blasts. Fusion hybrids induced anti-leukaemic T-cell immune responses in three of six patients. Tumour-pulsed mDC induced T-cellular responses in two other patients. Only one of six patients remission lymphocytes failed to develop leukaemia-directed immune responses following stimulation with either construct. Anti-proliferative properties of fusion hybrids against allogeneic lymphocytes were observed in mixed lymphocyte-leukaemia reactions and were found not to be specific to the cell fusion partners and did not prevent the ability of AML-mDC heterokaryons to induce autologous anti-leukaemic cytotoxicity. We conclude that tumour cell/dendritic cell fusion hybrids hold promise as a cellular vaccine for AML.

In vivo expansion of the endogenous B-cell compartment stimulated by radiation and serial bone marrow transplantation induces B-cell leukaemia in mice

Chronic lymphocytic leukaemia (CLL) is a malignancy of CD5(+) B cells. This B-cell lineage is established during ontogeny and replenished by the process of self-renewal. Spontaneous and induced leukaemias that frequently affect this lineage are thought to arise as a result of the frequent cell division required to maintain the population throughout adulthood and in response to repeated exposure to environmental antigens. In a series of bone marrow transplant (BMT) experiments performed in B6D2F1 mice, B-cell leukaemia occurred in recipients of serially transplanted syngeneic bone marrow. This study was therefore designed to determine the frequency and phenotype of the observed leukaemia. Male donor cells were initially transplanted into lethally irradiated female hosts and secondary (2 degrees ) BMT was performed at 3 months. At 1, 2, 3 and 16 months following primary (1 degrees ) BMT, and when 2 degrees BMT recipients developed leukaemia, animals were sacrificed and their tissues extensively examined. These analyses confirmed a host-derived CD5(+) transplantable B-cell leukaemia that was initiated in 50% of 1 degrees BMT recipients. With serial passage, the leukaemia became more aggressive and lost CD5 expression, suggesting transformation to a high-grade leukaemia/lymphoma. This previously unreported observation suggests that the combination of radiation and subsequent serial transplantation induces a proliferative stress to the host B-cell compartment that is causative in leukaemic transformation.

Pharmacological characterisation of pyrimidinoceptor responses in NG108-15 cells

In the present study, the P2Y receptor(s) mediating the effects of the pyrimidines UTP and UDP on phospholipase C activation in the mouse neuroblastoma x rat glioma hybrid cell line NG108-15 was investigated. Reverse Transcriptase-Polymerase Chain Reaction (RT-PCR) analysis detected transcripts for the P2Y(6) and P2Y(2) receptors, but not for P2Y(1) and P2Y(4.) UTP and UDP were equipotent agonists and their effects were partially additive. Suramin, reactive blue 2 and pyridoxal phosphate-6-azophenyl-2',4'disulfonic acid (PPADS) antagonised the phospholipase C response to both UTP and UDP. High micromolar concentrations of adenosine, 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS-21680), 2',3'-O-isopropylideneadenosine (iPAdo) and adenosine 3':5'-cyclic monophosphate (3',5'-cAMP) were able to antagonise the effect of UTP on phospholipase C but not that of UDP. The additivity of the UTP and UDP responses, novel P2 receptor antagonist profile and the distinguishing action of adenosine may indicate the expression of a pyrimidine selective P2Y receptor in addition to the P2Y(6) type in these cells.

Expression and activity of 2-5A synthetase in the course of differentiation and apoptosis of PC12 cells

The role of IFN-induced 2-5A system in cell differentiation has not been elucidated. While studying differentiation of PC12 cells we found that the simultaneous treatment of cells with NGF and IFN-gamma in serum-containing medium resulted first in the extension of neurites and then apoptosis. On the contrary, in serum-free medium the cells underwent a more rapid neuronal differentiation. Only the doses of NGF which induced the outgrowth of neurites from the cells were able to induce rapid cell death in combined treatment. When the cells were treated subsequently with NGF and IFN-gamma, the induction of death was observed with NGF post-treatment, but not with NGF pretreatment. Relying on these alternative biological responses, we studied the changes in 2-5A synthetase activity and its 43 kDa isoform expression in the course of differentiation and death of PC12 cells. The results of the present work showed that NGF-induced differentiation of the cells did not evoke any increase in 2-5A synthetase activity or any increase in the expression of its 43 kDa isoform. Moreover, the obtained results demonstrated that NGF could not significantly affect the IFN-induced signalling pathway leading to the activation of 2-5A synthetase gene, at least regarding the studied enzyme activity.

Formation of phase gratings on the end of gradient-index lenses with ultraviolet ablation at 193 nm

We present a simple, cost-effective method for creating diffractive optical elements on the surfaces of optical fibers and fiber-optic components by use of 193-nm ablation techniques. It is an outgrowth of a more fundamental investigation of the effects of intense UV radiation fields on SiO2- and Ge-SiO2-based structures (specifically optical fibers and preforms) and allows the inexpensive fabrication of structures such as the suggested evanescent-field-based sensing device.

Effect of neonatal treatment with a GnRH antagonist on development of the cell-mediated immune response in marmosets

PROBLEM

We examined the effect of neonatal treatment with a gonadotropin-releasing hormone (GnRH) antagonist (antide) on the development of cell-mediated immunity in male marmosets.

METHOD OF STUDY

Neonatal marmoset twins were treated with either vehicle or antide, and the proliferative response (PR) of lymphoid tissue to mitogens was assessed during infancy, the peripubertal period, and adulthood.

RESULTS

Basal proliferation of peripheral blood mononuclear cells (PBMC) from treated peripubertal twins was elevated above control values, but the PR of the cells to T and B cell mitogens was subnormal. Conversely, PBMC from treated infants exhibited an enhanced PR to some of the mitogens employed. In vitro culturing of thymocytes (control or treated) from the three developmental stages with either antide or a GnRH agonist increased basal proliferation, but decreased the PR to mitogens by 60-80%.

CONCLUSION

Neonatal treatment with antide alters development of, but does not permanently impair, cell-mediated immunity in the marmoset. GnRH appears to modulate immune responses throughout development in the primate.

Only pyrimidinoceptors are functionally expressed in mouse neuroblastoma cell lines

The ability of UTP, UDP, ATP, and ADP to influence inositol phospholipid hydrolysis in neuroblastoma origin cell lines was assessed. The mouse neuroblastoma lines N1E 115, Neuro 2a, and NB4 1A3 and the rat glioma/mouse neuroblastoma hybrid line NG108-15 gave robust responses to both UTP and UDP, which were essentially equipotent. Thus a range of cell lines of mouse neuroblastoma origin express a pyrimidine-selective P2Y receptor. The NG108-15 cells were the only cell type tested at which ATP and ADP displayed activity with EC50 values of greater than 100 microM, compared with values of 0.58 and 1.25 microM for UTP and UDP, respectively. In contrast to the cell lines derived from mouse neuroblastoma, the human neuroblastoma lines SH-SY5Y and SK-N-SH did not respond to any nucleotides, although both responded well to carbachol.

DNA ligase I is required for fetal liver erythropoiesis but is not essential for mammalian cell viability

Four distinct DNA ligase activities (I-IV) have been identified within mammalian cells. Evidence has indicated that DNA ligase I is central to DNA replication, as well as being involved in DNA repair processes. A patient with altered DNA ligase I displayed a phenotype similar to Bloom's syndrome, being immunodeficient, growth retarded and predisposed to cancer. Fibroblasts isolated from this patient (46BR) exhibited abnormal lagging strand synthesis and repair deficiency. It has been reported that DNA ligase I is essential for cell viability, but here we show that cells lacking DNA ligase I are in fact viable. Using gene targeting in embryonic stem (ES) cells, we have produced DNA ligase I-deficient mice. Embryos develop normally to mid-term when haematopoiesis usually switches to the fetal liver. Thereupon acute anaemia develops, despite the presence of erythroid-committed progenitor cells in the liver. Thus DNA ligase I is required for normal development, but is not essential for replication. Hence a previously unsuspected redundancy must exist between mammalian DNA ligases.

Skeletal dysplasias in a hospital in southern India

A hospital based study of skeletal dysplasias was conducted over a period of 2 years in Davangere, Karnataka, in which 169 cases of skeletal dysplasias were studied. One hundred were osteochondrodysplasias and were grouped according to international classification of osteochondrodysplasias. Among the individual cases, osteogenesis imperfecta (13 cases) had the maximum representation. Several cases of rare disorders were also identified. Eighty eight cases of skeletal dysplasias were in the pediatric age group and of these 41 were newborns. The incidence of skeletal dysplasia among newborns was 19.6 per 10,000 deliveries and lethal dysplasias 5.2 per 10,000 deliveries. In 7 cases of skeletal dysplasia, an antenatal diagnosis was possible by ultrasonography.

The effect of 9,11-secosterol, a newly discovered compound from the soft coral Gersemia fruticosa, on the growth and cell cycle progression of various tumor cells in culture

A new 9,11-secosterol, 24-nor-9,11-seco-11-acetoxy-3 beta,6 alpha-dihydroxycholest-7,22(E)-dien-9-one, was found to exhibit growth inhibitory (IC50 below 10 microM) and cytotoxic activities against human leukemia K562, human cervical cancer HeLa, and Ehrlich ascites tumor cells in vitro. The cytostatic concentrations of the compound generally caused the G2/M block in the cell cycle progression, but differences between the three tumor cell lines in the events leading to cell death were remarkable. While inhibiting cell proliferation, 9,11-secosterol caused accumulation of HeLa and K562 cells in the metaphase of mitosis. So, abnormal mitosis can play an important role in the cytotoxicity of 9,11-secosterol in these cell lines. In the Ehrlich ascites tumor cell line the increasing concentrations of the drug (up to 40 microM) did not cause an immediate cell killing. Instead, due to continued DNA synthesis without entry into mitosis, cells with high DNA ploidy were produced. It was shown that the cytoskeletal systems such as microtubules and microfilaments were not damaged by the action of 9,11-secosterol. Further studies are necessary to elucidate the mechanism of the cytotoxic effect of 9,11-secosterol.

Alternative pathways of apoptosis induced by methylprednisolone and valinomycin analyzed by flow cytometry

Apoptosis of murine thymocytes induced by either methylprednisolone or valinomycin was studied by flow cytometry. The apoptosis induced by methylprednisolone followed three stages: an initial decrease in cell volume, indicated by a fall in forward scatter accompanied by faint ethidium bromide staining, a second stage in which the cells became brightly stained by ethidium bromide, and a final stage when the cells were apparently less fluorescent as the nuclei disintegrated into apoptotic bodies. As the forward scatter of cells decreased there was a simultaneous depolarization of the cells and an elevation of intracellular calcium. These early changes preceded the fragmentation of the DNA which also preceded the intense staining of the cells by ethidium bromide. Methylprednisolone-induced apoptosis was inhibited by low concentrations (1 x 10(-7) M) of valinomycin and nonactin, neither of which could themselves induce apoptosis at these low concentrations. Cadmidazolium and cycloheximide arrested the program at an early stage. Okadaic acid allowed volume loss and ethidium bromide staining to proceed in the absence of DNA fragmentation. At high concentrations (1 x 10(-5) M) valinomycin induced a form of apoptosis, but nonactin only caused the cells to fragment. The valinomycin-induced apoptosis, although it involved the degradation of DNA and the disintegration of the nuclei into apoptotic bodies, differed from the methylprednisolone apoptosis as it did not involve a decrease of cell volume and was not inhibited by cycloheximide or affected by okadaic acid.

Age-dependent selection against hypoxanthine phosphoribosyl transferase-deficient cells in mouse haematopoiesis

The basis of a previously observed difference in the level of contribution of hypoxanthine phosphoribosyltransferase-deficient cells between the haematopoietic and non-haematopoietic tissues of chimaeric and heterozygous mice has been clarified by studying two populations of female mice that differ only in that one is heterozygous for a null allele at the hprt locus and the other is wild type at this locus. Both populations are heterozygous for an electrophoretic variant allele at the X-linked Pgk-1 locus, so that X-chromosome inactivation generates cells expressing different isozymes of phosphoglycerate kinase which can be assayed to monitor cell selection. The results show that hypoxanthine phosphoribosyltransferase deficiency itself, rather than an effect of another X-linked gene, causes a reduced level of contribution to haematopoietic tissues. Further, the extent of the depletion increases significantly with age, and this effect is due to a progressive reduction in the level of contribution to haematopoietic tissues rather than to an increase in the level of contribution to non-haematopoietic tissues.

Polycystic kidney disease in the CBA/N immunodeficient mouse

We describe a polycystic lesion of the kidney in the CBA/N mouse with an X-linked recessive immunodeficient syndrome. There is progressive cystic dilatation affecting all parts of the nephron. The cyst lining is composed of a single layered epithelium with focal nuclear crowding and the formation of micropapillary structures. The cystic epithelial cells show subnuclear vacuolation. Focal basement membrane thickening is also a feature. There is no significant inflammatory infiltrate present within these kidneys. Electron microscopic examination reveals that the subnuclear vacuolation is due to loss of the membrane infoldings at the basal pole of the epithelial cell with fluid accumulation within the extracellular space. The basement membrane thickening is due to expansion of the lamina densa. These changes are not present at birth but develop progressively with age. The finding of a polycystic kidney lesion in these mice offers an opportunity to investigate the relationship between the immune system and renal cyst formation.

Simultaneous analysis of immunophenotype and apoptosis of murine thymocytes by single laser flow cytometry

The study of the role of apoptosis in thymocyte development has been hampered by the lack of a means of directly immunophenotyping cells undergoing the early phase of apoptosis. This restriction has been overcome by single laser flow cytometry in which apoptosis is detected by Ethidium Bromide (EBr) staining and cell phenotype by binding of FITC-labelled antibody. The initial phase of apoptosis is observed as a cell population that stains faintly with EBr preceding the characteristically bright EBr-staining normally associated with cell death. Here we directly demonstrate using single laser flow cytometry that CD4+ CD8+ CD3low/CD3intermediate thymocytes undergo apoptosis in vitro in response to glucocorticoid treatment.

Hypoxanthine phosphoribosyl transferase deficiency, haematopoiesis and fertility in the mouse

We have looked for effects of deficiency in hypoxanthine phosphoribosyl transferase (HPRT) in the mouse comparable to non-behavioural consequences of HPRT-deficiency in humans. HPRT-deficient humans show abnormalities in haematopoiesis and, in heterozygotes, there is strong selection in haematopoietic tissues against HPRT-deficient cells arising as a result of X-chromosome inactivation. We have examined two situations in mice in which HPRT- and HPRT+ cells occur in the same individual. First, in chimaeras resulting from the injection of HPRT- embryonal stem cells into HPRT+ blastocysts the fate of HPRT- and HPRT+ cell populations was monitored by their expression of different isozymes of glucose phosphate isomerase and also, in those chimaeras that resulted from injecting the male ES cells into female blastocysts, by in situ hybridisation using a Y-chromosome-specific repetitive DNA probe. There was a small statistically significant selection against the HPRT- population in haematopoietic tissues in both XX in equilibrium with XY and XY in equilibrium with XY chimaeras. Second, in female mice doubly heterozygous for HPRT-deficiency and for an electrophoretic variant of the X-linked enzyme phosphoglycerate kinase, there was a similar small statistically significant selection against the HPRT- population in haematopoietic tissues. While further work is required to establish whether this selection is a consequence of the HPRT mutation, it is clear that any selection against cells in the haematopoietic system as a consequence of HPRT-deficiency is at most small compared with the effect seen in humans. In HPRT-deficient human males surviving beyond the normal age of puberty, there is testicular atrophy. However, we find no effect of HPRT-deficiency on the fertility of either male or female mice. Thus, as with effects on behaviour, the consequences of HPRT-deficiency for haematopoiesis and testis development in the mouse are at most small compared with those in the human. We conclude that the reason for the difference in effects between the two species lies in a difference in purine-related intermediary metabolism per se, rather than in its interaction with brain amine biochemistry.