ACVR1 (Activin Receptor Type 1) is a gene that encodes a receptor for activin, a protein involved in regulating cell growth and differentiation. This receptor plays a key role in signaling pathways that control how cells proliferate and develop. Mutations in ACVR1 are linked to fibrodysplasia ossificans progressiva (FOP), a rare genetic disorder marked by abnormal bone formation in soft tissues.