BMP4, as a member of the transforming growth factor-β (TGF-β) family binds to a 2 different types of serine-threonine kinase receptors known as BMPR1 and BMPR2 [REF].
Mitogen activated protein kinases (MAPK) undergo phosphorylation via a signaling cascade where MAPKKK phosphorylates and activates MAPKK and MAPKK phosphorylates and activates MAPK which then induces an intracellular response [REF].