Endothelial expression of a constitutively active Notch4 mutant from the Flk1 (VEGFR2) locus results in embryonic vascular abnormalities such as disorganized vascular networks and dilated blood vessels [33], and in adult mice, constitutively active Notch4 expression has been associated with arteriovenous malformations (AVMs) (i.e., abnormal connections between arteries and veins) that are accompanied by ectopic expression of the arterial marker ephrinB2 in veins [34]. The gene discussed is NOTCH4; the disease is arteriovenous hemangioma/malformation.