A cassette containing a tdTomato reporter followed by a floxed pgk-neo gene in inverse orientation was inserted in-frame just after the start codon in exon 1 replacing part of exon 1. Quantitative RT-PCR and immunoblot analysis confirmed the absence of expression of wild-type RNA and protein in the thymus and testis of homozygous mice. Reporter expression was detected in the cortical region of the thymus in heterozygous mice. (J:285040)