To date, only one clinical phenotype has been associated with a heterozygote germinal activating mutation in the Activin A receptor type 1 (ACVR1) gene, a component of the BMP receptor (BMPR), which is responsible for fibrodysplasia ossificans progressiva (FOP), an extremely severe and incurable, spontaneously arisen, progressive heterotopic ossification of soft tissues, mainly muscles and tendons. The gene discussed is ACVR1; the disease is fibrodysplasia ossificans progressiva.