BMP-8, also known as Osteogenic Protein 2 (OP-2) is a TGF-beta superfamily cytokine. BMP-8a is expressed during pregnancy in the deciduum and trophoblast cells, by inner root sheath cells of developing hair follicles, and by the epididymis and spermatids. In the mouse it cooperates with BMP-7 in the maintenance of spermatogenesis but is not required for the initiation of spermatogenesis. BMP-8b, in contrast, is required for both the initiation and maintenance of spermatogenesis. BMP-8a is induced during osteoblast differentiation at the onset of mineralization and during the osteogenic phase of bone repair in osteoblasts and osteocytes. BMP-8b is also highly expressed in osteosarcomas and pancreatic cancer, and it contributes to tumor progression. BMP-8b is also expressed in adipose tissue and the hypothalamus where it contributes to the regulation of energy balance and thermogenesis.