Activin is a powerful protein that regulates and participates in many cellular functions, including the menstrual cycle, and more specifically, the production and release of a reproductive hormone called follicle stimulating hormone (FSH). Activin is made by most organs and controls cellular proliferation and differentiation. Loss of activin can result in profound early developmental defects. Activin is regulated by a closely related protein called inhibin. Both activin and inhibin control male and female reproduction, and an imbalance in either of these hormones can result in infertility in both men and women.