A construct including GFP with a nuclear localization tag and an FRT flanked neo cassette was inserted in frame with the start methionine replacing a genomic fragment including most of exon 1 and all of intron 1 and exon 2. GFP is detected in the nuclei of PECAM+ endocardial and endothelial cells. Quantitative real time PCR and RT-PCR confirmed the absence of full length mRNA expression in tissues from homozygous mice. (J:187714)