The Bone Morphogenetic Protein (BMP) receptor activin receptor–like kinase 1 (Alk1), which is predominantly expressed in the vascular endothelium, has been shown to play a critical role in angiogenesis. Embryos lacking Alk1 die early during embryonic development due to impaired vascular remodeling and lack of perivascular cell coverage. In renal physiology, Alk1 has been suggested to play an important role in the regulation of extracellular matrix deposition, including collagen type I and fibronectin, and Alk1 heterozygosity has been associated with increased renal fibrosis in a mouse model of obstructive nephropathy, probably due to the decrease in the Alk1/Smad1 antifibrotic/protective signaling in renal fibroblasts.