RT-qPCR analysis of X-linked gene expression in single female XEN cells. (A) Cumulative histograms showing allelic gene expression levels of Xist, Atrx, Jpx, Ftx and Pbdc1 in the 72 individual extraendoderm stem (XEN) cells (GHP7/9 cell line). AU, Arbitrary unit. Logarithmic scale is used for Xist. (B) Cumulative histograms of the percentages of female XEN cells showing expression from the inactive paternal X (Xp) (dark blue), from the active maternal X (Xm) (red) or from both X chromosomes (light blue) for each indicated gene. X-linked genes are grouped according to their H3K27me3 enrichment in female XEN cells. The asterisks mark significant differences found upon analysis of the expression profile of a given gene showing low levels of H3K27me3 [K27-low] compared to any of the genes enriched in H3K27me3 [K27-high] analysed (P < 0.05 by Χ2 test). On the left, the diagram depicts the position of analysed genes along the X chromosome. The Pgk1a polymorphic region is shown in grey.