Mammalian X-chromosome inactivation is thought to be controlled by the X 
inactivation centre (XIC, X-controlling element -Xce-in mice). A human gene, 
XIST and its mouse counterpart, Xist, which map to the XIC/Xce, are expressed 
exclusively from inactive X chromosomes, suggesting their involvement in the 
process of X-inactivation. We now report the presence of Xist/XIST transcripts 
in newborn and adult mouse testes, and in human testicular tissue with normal 
spermatogenesis, but not in the testes of patients who lack germ cells. Our 
results indicate that while the X chromosome in males is active in somatic 
cells, it undergoes inactivation during spermatogenesis.