This document summarizes the molecular mechanisms and genetic consequences of X-chromosome inactivation. It describes how females equalize X-linked gene expression through randomly inactivating one of the two X chromosomes in their cells. The inactivation process is initiated by the XIST gene, which codes for a non-coding RNA that coats the inactive X and recruits chromatin modifiers to silence it. This results in cellular mosaicism in females and protects them from X-linked mutations while equalizing dosage between males and females.