An IRES-lacZ-PGK-neo cassette was inserted between exons 17 and 19 replacing exon 18 and its surrounding intronic sequences. Sequences encoding the first six transmembrain domains are replaced. The predicted protein is fused to a beta-galactosidase peptide and lacks the endogenous transmembrane and cytoplasmic domains. The absence of protein product was confirmed by western blot analysis on protein extracts from embryonic endothelioma cells established from E9.5 mice.