In humans, activating mutations in one of the Type I receptors, the Activin receptor IA/Activin-Like kinase 2 (ACVR1/ALK2), cause Fibrodysplasia ossificans progressive (FOP), a rare disease characterized by heterotopic endochondral ossification, through an increased BMP Smad-dependent and non-canonical p38 MAPK signaling activity [128]. Here, ACVR1 is linked to fibrodysplasia ossificans progressiva.