Inhibins/activins are involved in regulating a number of diverse functions such as hypothalamic and pituitary hormone secretion, gonadal hormone secretion, germ cell development and maturation, erythroid differentiation, insulin secretion, nerve cell survival, embryonic axial development and bone growth, depending on their subunit composition. Activin A is a disulfide linked homodimer (two beta-A chains) that binds to the two forms of activin receptor type I (Act RI-A and Act RI-B) and two forms of activin receptor type II (Act RII-A and Act RII-B). The biological activities of Activin A can be neutralized by inhibins and by the diffusible TGF-_ antagonist, follistatin.