BMP-15 (Bone morphogenetic protein 15), also known as GDF-9B, is a TGF-b superfamily ligand that is expressed by oocytes throughout folliculogenesis and plays an important role in oocyte development. BMP-15 promotes the FSH-independent proliferation of ovarian granulosa cells (GC) and induces GC glycolysis and cholesterol synthesis. BMP-15 blocks the FSH-induced GC expression of FSH R and multiple steroidogenic molecules. BMP-15 exerts is effects through interactions with BMPR-IB/ALK6 and BMPR-II, and nonphosphorylated BMP-15 functions as a competitive antagonist at BMPR-IB. Mature BMP-15 forms 34 kDa noncovalently-linked homodimers and 37 kDa heterodimers with mature GDF-9.