Mouse endogenous X-linked genes do not show lineage-specific delayed inactivation during development.

X chromosome inactivation (XCI) has been assumed to be complete in all cells of female mouse embryos at about 6 d post coitum (dpc). However, a recent study on beta-galactosidase expression of an X-linked lacZ transgene suggests that XCI is probably not complete several days after this time in some lineages. To help resolve this issue, we analysed XCI in embryos which carry the T(X;16)16H (Searle's) translocation and are heterozygous at the X-linked Hprt and Pgk-1 genes. The quantitative RT-PCR single nucleotide primer extension (SNuPE) assay was used to measure Hprt and Pgk-1 allele-specific transcripts in embryos 9.5 dpc. No transcripts from the normal X chromosome were found in any of the tissues tested, indicating that inactivation was complete for these endogenous genes.