There are three bone morphogenetic protein receptors in humans. Bone morphogenetic protein type I receptors are single pass, type I transmembrane proteins. They belong to a class of receptor serine/threonine kinases that bind members of the TGF beta superfamily of ligands-the Bone morphogenetic proteins. Bone morphogenetic protein receptor type II or BMPR2 is a serine/threonine receptor kinase. It binds Bone morphogenetic proteins, members of the TGF beta superfamily of ligands, which are involved in paracrine signalling.