TGFB1 and osteoarthritis: Transforming growth factor-β (TGF-β) and bone morphogenetic protein (BMP) play important roles in bone metabolism.1–3 Genetic mutations in TGF-β or BMP signaling pathway components cause heritable developmental bone diseases, and dysregulation of TGF-β or BMP signaling is often associated with osteoporosis or osteoarthritis.2,4,5 Through binding to membrane-localized serine/threonine kinase receptors, these ligands elicit specific signals that activate intercellular signal cascades, including those mediated by Smad proteins.