Exons 14 and 15, which together encode the transmembrane IIS5 segment and the pore in repeat II, were flanked by loxP sites and subsequently excised in vitro cre-mediated recombination prior to germline transmission. Though RT-PCR revealed aberrant transcript in which exon 13 had spliced to a point within intron 16, no protein was detected in the hearts of homozygous mutant embryos by Western blot analysis.