D, Persistent Ki67 expression in the epithelium of Rb1d/d mice on day 4 was recovered by pre-implantation P4 treatment on days 2 and 3. Scale bar = 100μm; le, luminal epithelium; s, stroma. E, Ratio of Ki67-positive epithelial cells in Rb1d/d mice was completely suppressed by pre-implantation P4 supplementation on days 2 and 3. The same data of Rb1f/f and Rb1d/d mice without P4 treatment in Fig 2D were used (mean ± SEM, Student's t test; n=5 different mice for each group). Three different high-powered fields per mouse were analyzed and each of the Ki67-posivie ratios was demonstrated. 