A loxA1 site was inserted upstream of exon 1. An FRT-flanked neomycin resistance cassette with a 5' loxA1 site was inserted in intron 1. Flp-mediated recombination removed the neomycin resistance cassette. Cre-mediated recombination removed exon 1. Immunohistochemistry confirmed the absence of protein expression in the sinoatrial node. (J:179977)