ACVR1 and fibrodysplasia ossificans progressiva: In 2006, a rare genetic disorder involving the skeletal muscle, called fibro-dysplasia ossificans progressiva (FOP; OMIM #135100), was reported to be associated with a recurrent mutation in the Activin A receptor type I (ACVR1) gene, which encodes the transmembrane receptor Activin receptor-like kinase 2 (ALK2)/ACVR1 protein [1] (Figure 1).