SMAD1 and fibrodysplasia ossificans progressiva: The association with the Activin and TGFβ signaling pathways indicates signaling via the non-mutated receptor, whereas the BMP signaling via SMAD1, 5, and 9 shows that the altered responsiveness of the mutated receptor to Activin-A as described by other groups [20,52] is also present in the periodontal ligament fibroblasts from FOP patients.