Exon 1 and approximately 1.5 kb of sequence upstream of exon 1 (promotor region) was flanked by loxP sites. A FRT-flanked puromycin resistance gene was inserted into intron 1 and removed via Flp-mediated recombination. Cre-mediated recombination removed the floxed region deleting the promotor region and transcriptional and translational start sites. Western blot analysis of testis lysates confirmed absence of protein in homozygotes. (J:277272)