An FRT-flanked neo cassette with a 5' loxP site was inserted upstream of the translation initiation site in exon 1. An additional loxP site was inserted downstream of exon 2. Cre-mediated recombination removed the neo cassette, 3' portion of exon 1 downstream of the translation initiation codon, intron 1, and exon 2. Western blot analysis confirmed the absence of protein expression in thymocytes.