A loxP site was inserted in intron 4 and an FRT flanked neo cassette and loxP site were inserted in intron 6 via homologous recombination. Cre mediated recombination removed exons 5 and 6 and the neo cassette. Homozygous mice express a shorter mRNA form at levels similar to expression of the full length form in wild-type mice. (J:171655)