ACVR1 and fibrodysplasia ossificans progressiva: The most common cause (>98 %) is Fibrodysplasia Ossificans Progressiva (FOP), a condition of extra skeletal bone formation caused by gain-of-function variants in the ACVR1 gene, which encodes a bone morphogenetic protein (BMP) type 1 receptor (Activin A Receptor Type 1) [3, 4].