FLT4 and neoplasm: VEGFR3 expression is essential for angiogenesis in early mouse embryos (Dumont et al, 1998), but outside the CNS, it is down-regulated in systemic blood vessels when the lymphatic system begins to develop around E10.5, and by E16.5 its, expression is largely confined to lymphatic vessels, although it is reactivated in tumor blood vessels (Valtola et al, 1999; Tammela et al, 2008) and after injury (Paavonen et al, 2000).