DLL4 and isolated hemihyperplasia: VEGF‐A165b, the most investigated of the anti‐angiogenic variants, plays a key role in regulating both cell proliferation and disrupting/preventing the up‐regulation of DLL4 and establishment of the tip and stalk endothelial phenotype, demonstrating for the first time the relative proportion of pro‐ and anti‐angiogenic VEGF‐A isoforms as a potential mediator of IH involution.