ACVR1 and fibrodysplasia ossificans progressiva: Notably, even though FOP is caused by gain-of-function mutation of the type 1 bone morphogenetic protein (BMP) receptor,9,10 ACVR1 (also known as ALK2), the initiation of the HO process in FOP is similarly triggered by abnormal immune responses to minor injuries (also called flare-up) followed by persistent low-grade inflammation.