Figure 3.

Typical chrX states in female human embryo development. Human genome activation and expression from the X chromosomes occurs by the 8-cell stage (far left). In the preimplantation embryo (left), gene expression occurs from both of the X chromosomes, which are coated with the long noncoding RNAs XIST and XACT. Gene expression is modulated through chrX dampening. Sometime after implantation (middle), random XCI occurs. Gene expression becomes restricted to only Xa, and Xa is coated with XACT. Xi is coated with XIST, and gene expression is silenced. In somatic cells, Xi remains silenced except for escapee genes. Random XCI is preserved in somatic cells (box), resulting in X-chromosomal mosaicism in females, where a subset of cells will express the maternal allele of a gene, and the remaining cells will express the paternal alleles of a gene. chrX, X chromosome; G, gene; M, maternal; P, paternal; Xa, active chrX; XCI, X chromosome inactivation; Xi, inactive chrX.