A single loxP site remained in place of exon 3 after an upstream floxed neo cassette and downstream loxP site were recombined by transient expression of cre recombinase in targeted ES cells. The mutation results in deletion of the transmembrane domain and the GS domain required for protein function. Northern blot analysis using a full-length cDNA probe did not detect transcripts in endothelial cells from homozygous mutant embryos.