ABSTRACT
The basis of a previously observed difference in the level of contribution of hypoxanthine phosphoribosyltrans-ferase-deficient cells between the haematopoietic and non-haematopoietic tissues of chimaeric and heterozy-gous mice has been clarified by studying two populations of female mice that differ only in that one is heterozy-gous for a null allele at the hprt locus and the other is wild type at this locus. Both populations are heterozy-gous for an electrophoretic variant allele at the X-linked Pgk-1 locus, so that X-chromosome inactivation generates cells expressing different isozymes of phos-phoglycerate kinase which can be assayed to monitor cell selection. The results show that hypoxanthine phospho-ribosyltransferase deficiency itself, rather than an effect of another X-linked gene, causes a reduced level of con-tribution 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 con-tribution to haematopoietic tissues rather than to an increase in the level of contribution to non-haematopoi-etic tissues.
