Using induced pluripotent stem cells (iPSCs) established from FOP patients (FOP-iPSCs), we identified Activin A as a crucial molecule that transduces the BMP signal from mutated ACVR1–containing receptor complexes in mesenchymal stem cells induced from FOP-iPSCs (FOP-iMSCs) (Hino et al, 2015). The gene discussed is ACVR1; the disease is fibrodysplasia ossificans progressiva.