An important ovarian hormone produced by the ovaries following ovulation during the menstrual cycle. Progesterone rises 1400 percent over a week from the midcycle to its peak during the luteal phase. It acts on specific receptors in every tissue in the body in which estrogen acts. Its primary job is to cause differentiation (maturation) and to stop the proliferation (growth) caused by estrogen. Progesterone causes the endometrium to become secretory and able to accept and nurture a fertilized egg. If fertilization does not occur estrogen and progesterone levels decrease and a period results.

Refers to cells in breast cancer that are stimulated by estrogen to grow and undergo proliferation.