BMP ligands can form homo- or heterodimers which signal through a tetrameric set of receptors, comprising two type I and two type II receptors. Binding of BMP dimers to the receptors triggers a phosphorylation cascade that result in phosphorylation of Smad 1/5/8 and players of various non-SMAD pathways to initiate SMAD and non-SMAD signaling cascades. The non-SMAD pathways include signaling by mitogen-activated proteins kinases (MAPKs p38, ERK, and JNK), PI3K/AKT, and small Rho-like GTPases. Phosphorylated Smads are able to form complexes with Smad4 and translocate to the nucleus. (Illustration credit: Ben Smith)