A, Rb1 protein (RB) was expressed in uterine luminal epithelium and stroma during early pregnancy in mice. Scale bar = 100μm; arrowhead, embryo. B, Phosphorylated RB (pRB) protein, the inactive form of RB, was expressed in the luminal epithelium on days 2 and 3 of pregnancy. Scale bar = 100μm. 