ACVRL1 and anemia (phenotype): Furthermore, we and others have shown that endothelial-specific, or total, depletion of ALK1 in adult life leads to caecal bleeding, anemia, and rapid mortality.21,53,54 Pelvic AVMs were not reported in these studies but may have been present, although challenging to dissect the relative contribution of anemia and AVMs on heart pathophysiology of Alk1-iKO mice, while their rapid mortality limits longitudinal studies of disease progression and therapeutic interventions.