Progesterone (P4) is an an intermediate of sex steroids produced by all steroidogenic tissues. It maintains pregnancy and the secretory condition of the uterine endometrium during the luteal phase, and inhibits release of gonadotropins. P4 can bind to progesterone receptor (PR), which is a member of the nuclear receptor superfamily of transcription factors. PR has two natural isoforms, PRA and PRB, which are transcribed from the same gene. Since PRA knockout (PRAKO) mice exhibit severely reduced ovulation rates and implantation defects, PRA is responsible for many functions in the reproductive organs including uterus. PRB is critical for mammary gland branching and ductal development in mice.