Primary non-random X-inactivation caused by controlling elements in the mouse demonstrated at the cellular level

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Abstract
SUMMARY: Previous studies have shown that different alleles of the mouseXchromosomal controlling element locus,Xce, cause non-randomX-chromosome inactivation as judged by variegation in the coats of female mice heterozygous for X-linked coat colour/structure genes, or Cattanach's translocation (Is (X; 7) Ct), or the relative activity of biochemical variants of theX-linked enzyme PGK. This paper presents evidence using the Kanda differential staining method on 6½ d.p.c. and 13½ d.p.c. female mouse embryos heterozygous for the markerXchromosome Is (X; 7) Ct and carrying differentXcealleles, that theXcelocus affects the randomness ofXchromosome inactivation. Furthermore the fact that a markedXceeffect is demonstrable in female embryos as early as 6½ d.p.c. (i.e. very soon after the initial time ofX-inactivation) is strong evidence that theXcelocus exerts its effect by causing primary non-randomX-inactivation rather than by cell selection after initial randomX-inactivation.