A genomic fragment encompassing exons 1 and 2 was replaced with a floxed neo cassette inserted by homologous recombination. RT-PCR analysis showed the presence of the alternative transcript composed of exon 1b spliced directly to exon 3 which is the endoplasmic reticulum isoform, but absence of the mitochondrial specific isoform, indicating this mouse is an isoform-specific knock-out.