H, Number of pups delivered by Rb1d/d dams was normalized by pre-implantation P4 treatment on days 2 and 3 (Group 2)(mean ± SEM, Student's t test). The same data of Rb1f/f and Rb1d/d dams without P4 treatment in Fig 1E were used. The numbers of pups in 20 Rb1f/f and 37 Rb1d/d dams without P4 treatment and 7 Rb1d/d dams with P4 treatment were evaluated. 