PRL is essential for lactation in all mammals, although the precise temporal dimensions of its actions vary among species. The first step in mammary gland organogenesis is the prenatal establishment of the mammary ductal rudiment. Parathyroid hormone-related peptide (PTHrP) is essential at this first stage of mammary gland development, but PRL is not.^[83,85,113] The epithelial rudiment and the fat pad grow isometrically until puberty, at which time the epithelial ductal system expands rapidly under the influence of estrogen, GH, and IGF-1.^[114,115]During the latter stages of puberty, lobular buds branch off from the ductal system under the influence of PRL and progesterone. As a consequence of the regular cycles of estrous or menstrual hormone surges, the complexity of mammary ductal branching increases progressively, and the epithelial cells undergo cyclic changes. If the female becomes pregnant before lobule …