Exon 2, which encodes the initiator methionine and the N-terminal signal peptide, was replaced with a neo cassette. Transcript encoding the heart and kidney isoform was undetected in homozygous mutant mice, however truncated transcript similar to the testis-specific isoform was detected. RT-PCR analsysis indicated that the mutant transcript lacked exon 2. The absence of normal protein was confirmed by FACS analysis of erthrocytes obtained from homozygous mutant mice.