Expression of the X inactive-specific transcript (Xist) is thought to be 
essential for the initiation of X chromosome inactivation and dosage 
compensation during female embryo development. In the present study, we analyzed 
the patterns of Xist transcription and the onset of X chromosome inactivation in 
bovine preattachment embryos. Reverse transcription-polymerase chain reaction 
(RT-PCR) revealed the presence of Xist transcripts in all adult female somatic 
tissues evaluated. In contrast, among the male tissues examined, Xist expression 
was detected only in testis. No evidence for Xist transcription was observed 
after a single round of RT-PCR from pools of in vitro-derived embryos at the 2- 
to 4-cell stage. Xist transcripts were detected as a faint amplicon at the 
8-cell stage initially, and consistently thereafter in all stages examined up to 
and including the expanded blastocyst stage. Xist transcripts, however, were 
subsequently detected from the 2-cell stage onward after nested RT-PCR. 
Preferential [3H]thymidine labeling indicative of late replication of one of the 
X chromosomes was noted in female embryos of different developmental ages as 
follows: 2 of 7 (28.5%) early blastocysts, 6 of 13 (46.1%) blastocysts, 8 of 11 
(72.1%) expanded blastocysts, and 14 of 17 (77.7%) hatched blastocysts. These 
results suggest that Xist expression precedes the onset of late replication in 
the bovine embryo, in a pattern compatible with a possible role of bovine Xist 
in the initiation of X chromosome inactivation.