SMAD1 and neoplasm: INHA has recently been suggested as a novel paracrine factor for tumor angiogenesis and metastasis based on in vitro experiments demonstrating that tumor-cell-derived INHA can induce the growth of cultured endothelial cells through a signaling pathway involving the TGF beta co-receptor endoglin and its downstream activators of angiogenesis, ALK1 and SMAD1/5 [44].