Molecular model of a mutant copy of the activin A receptor type I (ACVR1) protein (brown and light pink) that causes the rare genetic disease fibrodysplasia ossificans progressiva (FOP). FOP is a disease where fibrous tissues, including muscle, tendons and ligaments, are gradually replaced by bone (ossified). A mutation in the ACVR1 gene, which plays a role in the bone morphogenic protein (BMP) pathway, leads to a change in amino acid at position 206, where arginine is swapped for histidine (rod shaped). This decreases the affinity of the ACVR1 protein for its inhibitor FK506-binding protein 12 (FKB12, yellow), leading to increased activity of the protein and therefore increased production of bone.