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Bicuspid aortic valve (BAV) is a common congenital heart defect (population incidence, 1–2%) 1 – 3 that frequently presents with ascending aortic aneurysm (AscAA) 4 . BAV/AscAA shows autosomal dominant inheritance with incomplete penetrance and male predominance. Causative gene mutations (for example, NOTCH1 , SMAD6 ) are known for ≤1% of nonsyndromic BAV cases with and without AscAA 5 – 8 , impeding mechanistic insight and development of therapeutic strategies. Here, we report the identification of variants in ROBO4 (which encodes a factor known to contribute to endothelial performance) that segregate with disease in two families. Targeted sequencing of ROBO4 showed enrichment for rare variants in BAV/AscAA probands compared with controls. Targeted silencing of ROBO4 or mutant ROBO4 expression in endothelial cell lines results in impaired barrier function and a synthetic repertoire suggestive of endothelial-to-mesenchymal transition. This is consistent with BAV/AscAA-associated findings in patients and in animal models deficient for ROBO4. These data identify a novel endothelial etiology for this common human disease phenotype. Individuals with biscuspid aortic valve and ascending aortic aneurysm show enrichment of rare variants in ROBO4 . Functional studies suggest that ROBO4 mutations disrupt endothelial cell performance and contribute to pathological remodeling of aortic tissues.