X-inactivation is a process by which genes along one of the two X-chromosomes in
females undergo silencing. This phenomenon equalizes X-linked gene expression in
male and female mammals. It’s widely believed that all cells undergo X-inactivation
by the blastocyst stage of embryogenesis in females. Moreover, X-inactivation is
posited to be triggered by the expression of the X inactive-specific transcript (Xist)
from (paradoxically) the inactive-X. However, this image of a female mouse
blastocyst shows that 1) Xist RNA (in green; the RNA, unusually, decorates most of
the inactive-X) is not detectable in some cells; and, 2) some cells show biallelic
expression of the X-linked gene Atrx (red pinpoint dots of nascent RNA expression)
despite the presence of Xist RNA on the inactive-X. Thus, X-inactivation is
incomplete at the blastocyst stage. Nuclei are stained blue by DAPI.