 It is thought that this constitutes the mechanism of choice, and allows downstream processes to establish the compact state of the Barr body.These changes include histone modifications, such as histone H3 methylation (i.e.H3K27me3 by PRC2 which is recruited by Xist) and histone H2A ubiquitination, as well as direct modification of the DNA itself, via the methylation of CpG sites.