ACVR1 and fibrodysplasia ossificans progressiva: In related studies, others showed that small molecule antagonists of BMP signaling [56, 57] inhibited ectopic cartilage and bone formation in mouse models of the severe pediatric disorder Fibrodysplasia Ossificans Progressiva (FOP) [58] that is caused by activating mutations in the type I BMP receptor ACVR1/ALK2 [59].