Figure 2

X chromosome inactivation and regulatory mechanisms. (A) The Xic region of the X chromosome and several regulatory elements in this region. Xist loci encodes a ncRNA which mediates the initiation of XCI. Xist is negatively regulated by another ncRNA, Tisx, which is complementary to Xist. Xite is a special upstream enhancer of Tisx. (B) By pairing at the Xic region, X chromosomes communicate and counting, selecting are then undergone to make sure that only one X chromosome remains active. Xa, active X chromosome; Xi, inactive X chromosome. (C) Mechanisms from initiation of XCI to the short- and long-term maintenance of silencing status. After selection, Xist coats the Xi chromosome to initiate XCI. A ncRNA (RepA) then usher polycomb repressive complex 2 (PRC2) to the Xi chromosome, recruiting histone modification changes such as H2A-K119 ubiquitination (U) and H3K27 methylation (M). Histone modifications bring about short-term silencing followed by histone variant incorporation (macroH2A) and de novo DNA methylation, mediating long-term silencing.