Progesterone is another ovarian steroid hormone which plays a key role in mammary development. While progesterone receptors have been difficult to identify in mammary fat pad, administration of progesterone can result in proliferation of stromal cells under some physiological conditions. The stimulatory effect of progesterone on DNA synthesis in ductal epithelium is probably mediated indirectly through its effects on stromal cells. The major mammogenic effect of progesterone is mediated through binding to its receptors in epithelial cells and stimulating ductal sidebranching or alveolar bud formation, which are the hallmarks of postpubertal mammary development. Estrogen stimulation of progesterone receptor expression in epithelial cells is required for this progesterone effect. Progesterone, therefore, has a major role in alveolar morphogenesis and a lesser role in ductal morphogenesis. During estrous cycles, duct elongation and expansion of the parenchymal tissue into the fat pad occur in limited bursts associated with the period of elevated estrogen. During the luteal phase of elevated progesterone in ruminants, relatively little further expansion occurs, but formation and maintenance of lobular structures may be stimulated by progesterone, with little ductal regression occurring between cycles.