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RU 486 is the patented name for a drug known scientifically as "mifepristone." It was developed in 1980 by the French pharmaceutical company Roussel Uclaf (hence the "RU"). Taken orally, it simply prevents the hormone progesterone from doing its part in maintaining a pregnancy.

In the first half of a woman's ovulatory cycle, estrogens secreted by the ripening egg follicle (prompted by hormones from the brain) cause the wall of the uterus to start thickening. Near the middle of the cycle, when estrogens are at their peak, a surge of the hormone called luteinizing hormone cues the follicle to release its egg. The cells left behind form the corpus luteum, which secretes progesterone. Progesterone prepares the womb for pregnancy.

In the preparation process, known as decidualization, the lining of the womb becomes thicker and the blood supply to it increases. An embryo will typically attach to the uterine wall about 2 weeks after the egg has been fertilized. After 14 days, unless a fertilized egg implants in the womb, the corpus luteum disintegrates, the level of progesterone drops, and the lining of the womb is expelled [in menstruation]. ⁵

"If there is an embryo, cells around it secrete a hormone called human chorionic gonadotropin, which rescues the corpus luteum, keeping it active and secreting progesterone. As a result the cells lining the uterus, called the endometrium, stay in place and accept the implantation of the embryo." ⁵ More specifically, the progesterone binds to a receptor, a molecule which recognizes specific hormones the way a lock recognizes a key. ⁷ The receptor in turn is already bound to a particular kind of protein called a heat shock protein. The act of the progesterone binding to the receptor changes the shape of the receptor in a way that releases the heat shock protein. The area of the receptor formerly occupied by the protein is then free to bind to areas on a cell's DNA called hormone response elements. "That step, in turn, alters the DNA so that the genes controlled by progesterone can be transcribed." ⁵

RU 486 binds to the receptor in exactly the same fashion as progesterone does, but without releasing the heat shock protein; in fact, the protein may become even more tightly bound. "As a result, the receptor is unable to bind to the hormone response elements, and no transcription of the DNA takes place. RU 486 occupies the receptors, preventing progesterone from binding to them, and any processes that depend on progesterone, such as the maintenance of pregnancy, fail." ⁵

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