The genetic analysis of Vezf1 (vascular endothelial zinc finger 1) function during the formation of the murine vascular system is described in this study. The analysis of Vezf1 function by gene inactivation (gene KO) in mice revealed that Vezf1 acts in a tightly regulated dose- as well as strain-dependent fashion during the development of the blood vascular and lymphatic system. Lack of Vezf1 leads to angiogenic remodeling defects and compromised integrity of the embryonic vasculature. Furthermore, heterozygous embryos display lymphatic hypervascularization associated with hemorrhaging and edema localized to the jugular region, a phenotype very reminiscent of the human congenital malformation syndrome cystic hygroma. In addition, it was demonstrated that the transgenic overexpession of Vezf1 in the blood vasculature in the homozygous state results in hypervascularization in snouts, ears and skin, underscoring the importance of Vezf1 as a regulator of vascular development and the dose-dependency of its function.