Subsequent studies of the TGF-β superfamily revealed additional genes responsible for PAH: activin receptor-like kinase 1 (ALK1), endoglin (ENG), SMAD1/4/8, and bone morphogenetic protein receptor 1B (BMPR1B) (Trembath et al. 2001; Harrison et al. 2003, 2005; Shintani et al. 2009; Chida et al. 2012; Nasim et al. 2012). The gene discussed is SMAD1; the disease is pulmonary arterial hypertension.