Cellular responses to bone morphogenetic proteins (BMPs) are mediated by the formation of hetero-oligomeric complexes of type I and type II serine/threonine kinase receptors. There are seven known type I serine/threonine kinases and five mammalian type II serine/threonine kinase receptors that function in TGF-beta superfamily signal transduction. The type II receptors for TGF-beta and activin bind ligands with high affinity by themselves. In contrast, BMPR-II binding to BMP-2, BMP-4, and BMP-7 is facilitated by the presence of the type I receptor. The type I receptors are also known as activin receptor-like kinases or ALKs.