The endothelial specific receptor tyrosine kinase promoter was used to drive expression of cre recombinase. Nine founder lines were produced and one of these was selected for use that efficiently mediated gene excision. Cre was active in the blood islands of extraembryonic mesoderm at E7.5 and in the dorsal aortae at E8.5. By E11.5 it was active in the endothelial cells of all blood vessels.