Actions of P4 in reproduction are mediated in large part by the classical nuclear receptor, Pgr, as shown by infertility and loss of most P4 functions in female progesterone receptor knockout (PRKO) mice lacking nuclear Pgr (Lydon et al., 1995; Conneely et al., 2001; Jeong et al., 2005). Here, PGR is linked to Infertility.