Activin A, a member of the transforming growth factor-beta (TGF-β) superfamily. In FOP, patients have a gain-of-function mutation in the *ACVR1/ALK2* gene, which encodes a bone morphogenetic protein (BMP) type I receptor. This mutation fundamentally alters the receptor's behavior: it misinterprets Activin A—a protein that normally acts to inhibit BMP signaling—as a potent activator. When Activin A binds to the mutant ACVR1 receptor, it incorrectly triggers a powerful osteogenic (bone-forming) signal. This aberrant signaling cascade leads to endochondral ossification, the process responsible for the catastrophic and progressive bone formation in FOP.