In this study, we evaluated the proliferation of MSCs obtained from FOP patient-derived induced pluripotent stem cells (FOP-iPSCs) to elucidate the mechanisms underlying flare-ups and found that bone morphogenetic protein (BMP)-9 mediated enhanced proliferation by abnormal activation of transforming growth factor (TGF)-β signaling pathway in MSCs from FOP-iPSCs. The gene discussed is GDF2; the disease is fibrodysplasia ossificans progressiva.