Comparison of male and female gene expression in mammals. In mouse (a) and humans (b), each tissue has a distinct distribution of M:F ratios, but in each case the distribution for X genes (red line) fits closely to the distribution for A genes (dotted black line). LB, lymphoblastoid cell lines. PBM cells, peripheral blood mononuclear cells. Arrows point to regions where the X and A curves diverge, or to the inflection point in the mouse adipose tissue curve.