X chromosome activity in female germ cells of mice heterozygous for Searle's translocation T(X;16)16H

SUMMARY The expression of X-linked phosphoglycerate kinase (PGK-1) in germ cells from embryos heterozygous for both PGK-1 and Searle's translocation T(X; 16) 16H was examined to investigate X chromosome activity during oogenesis. The Pgk-l b allele on the translocated X chromosome was the only allele active in somatic cells of all embryos and in germ cells from 12·5 d.p.c. embryos. However, an additional faint band representing Pgk-l a activity was observed in germ cells from older embryos (13·5–18·5 d.p.c.) and neonates (1–2 d.p.p.). It is concluded that there is a period when only one X chromosome is active in early female germ cells and that reactivation of the inactive X chromosome takes place just prior to meiotic prophase.