ACVR1 and fibrodysplasia ossificans progressiva: In most FOP patients, the underlying cause is a single nucleotide c.617G > A mutation that substitutes arginine for histidine at Codon 206 (R206H of the BMP Type 1 receptor called Activin receptor Type 1/Activin Kinase 2 (ACVR1/ALK2) [2].