A naturally occurring hormone that plays an important role in fetal development may be the basis for a new type of reversible contraceptive that can protect ovaries from the damage caused by chemotherapy drugs. In their report receiving online publication in PNAS, a team from the Pediatric Surgical Research Laboratories in the Massachusetts General Hospital (MGH) Department of Surgery describes using Mullerian Inhibiting Substance (MIS) to halt, in a mouse model, the early development of the ovarian follicles in which oocytes mature, an accomplishment that protects these primordial follicles from chemotherapy-induced damage.