To determine whether arterial identity is a prerequisite for aortic HE, we analysed MO-mediated repression of dll4 and indeed found a loss of runx1 expression (Fig. 6e), as well as of cmyb that could be rescued by overexpressing runx1 in dll4 morphants (Supplementary Fig. 9b), while endothelial integrity was unaffected (Supplementary Fig. 9c). The gene discussed is RUNX1; the disease is hereditary elliptocytosis.