Exons 5 and 6 were replaced with a floxed Pgk-neo cassette via homologous recombination. Cre mediated recombination removed the neo cassette. RT-PCR analysis confirmed the absence of the short, predominant, 1500 bp transcript but detected the presence of a second 1215 bp transcript that skips exons 5 and 6 and contains a novel exon 7. However, western blot analysis did not detect either protein product.