BMPR1A and pulmonary arterial hypertension: Murine homozygous null mutants for BMPR-1a (Activin like kinase 3, ALK3) or the type II receptor BMPR-2 (which is mutated in human patients with PAH) [22,23] and their ligand Bmp4 or downstream co-Smad4 are embryonic lethal, and present with vascular deformities attributable to a loss of Smad1 mediated signalling [24].