Researchers at University of Michigan show that a molecule called Xist RNA is insufficient to silence the X chromosome. “Xist is widely believed to be both necessary and sufficient for X silencing,” said assistant professor Sundeep Kalantry who led the study .
Now, the researchers have shown that Xist has to have accomplices. They reside in the X chromosome that is destined to get silenced. Although most genes on the inactive X chromosome are fully silenced, a handful of the genes on the inactive X are in fact active. It is this set of X inactivation ‘escapees’ that the researchers focused on.
Since the ‘escapee’ genes are expressed from both the active and the inactive Xchromosomes in females, they produce more gene product in female cells than in male cells, which only have a single X. The researchers predict that it is this higher ‘dose’ in females that triggers Xinactivation selectively in females; the lower dose in males is insufficient.