High-resolution Xist binding maps reveal two-step spreading during X-chromosome inactivation

During mammalian X-chromosome inactivation, the Xist long noncoding RNA coats the future inactive X chromosome and recruits polycomb repressive complex 2 to a nucleation site, but how Xist spreads silencing across the entire X chromosome is unclear; here high-resolution maps of Xist binding sites across the X chromosome are generated and show that Xist does not spread across the inactive X chromosome uniformly but in two steps, initially targeting gene-rich islands before later spreading to intervening gene-poor domains.