Binding of a BMP dimer to its type II receptor recruits type I receptors, so that a heterotetramer is formed with two receptors of each type. The proximity of the receptors allows the type II receptor to phosphorylate the type I receptor. One of two identified downstream pathways, the Smad cascade, is initiated by phosphorylation of certain Smad proteins by type I receptors, and the other pathway involves two mitogen-activated protein kinase (MAPK) cascades. In either case, the consequence is regulation of gene transcription.