X inactivation-specific transcript expression in mouse oocytes and zygotes

Expression of the X inactivation-specific transcript (Xist) gene has previo usly been shown by reverse transcription-polymerase chain reaction (RT-PCR) to be present at the 4-cell stage of female mouse embryos. This early expr ession, which is followed by X inactivation in the extra-embryonic tissues, is maternally imprinted. By the blastocyst stage, as the embryonic lineage s begin to form, the imprint is lost and expression becomes random. By appl ying in-situ RT-PCR, we showed that Xist is expressed even earlier in devel opment, in unfertilized mouse oocytes as well as in pronuclei stage zygotes . Our data demonstrate Xist expression in oocytes and suggest that Xist tra nscripts may occur in both XX and XY zygotes. A difference in the pattern o f expression (rod-like or rounded punctate signal) is found among pronuclea i-stage embryos. Early expression is in agreement with findings reported in human embryos.