Bone morphogenetic proteins (BMPs) belong to the transforming growth factor beta (TGF-β) superfamily. BMPs play a key role in embryonic development, especially during heart, neural, and cartilage development. Around 20 BMP family members have been identified and characterized. BMPs signal through serine/threonine kinase receptors, composed of type I and II subtypes. Four type I receptors have been identified: type IA and IB BMP receptors, type IA activin receptor, and activin receptor-like kinase I (ALK-1). Three type II receptors have also been recognized: type II BMP receptor and type II and IIB activin receptors. Human recombinant BMP-6 can restore bone in animal models of osteoporosis. BMP-6 induces bone formation through IGF-I and EGF pathways in a murine model of osteopenia and in human osteoblasts. BMP-6 has been reported to have an antiproliferative effect on B, T cells, and macrophages. Also, BMP-6 induced expression of pro-inflammatory inducible nitric oxide synthase (iNOS), TNF-α and IL-1β in macrophages. In addition, BMP-6 is a key endogenous regulator of hepcidin expression and iron metabolism in vivo. In fact, BMP-6 null mice have a phenotype that resembles mouse models of juvenile hemochromatosis (an iron overload disorder caused by mutations in the genes encoding the major iron regulatory hormone hepcidin and hemojuvelin).