A gene trap vector (pGT1TMpfs) insertion resulted in the fusion of the first Ig domain to TMbetageo, an artificial protein created from the transmembrane domain of rat Cd4 joined to the catalytically active domains of betagalactosidase and neomycin genes followed by an IRES and placental alkaline phosphatase reporter cassette. The betagalactosidase and neomycin genes are flanked by Frt sites. The absence of protein product in the endothelium of the aorta was confirmed by immunohistochemistry.