A self-excising cassette, containing a tACE promoter, a cre gene, HSV-TK poly(A) sequences and a neomycin resistance gene cassette, was used to target the promoter. This resulted in a 644 bp deletion encompassing the promoter and exon 1. The deleted sequence is also part of intron 2 of the Rag2 gene. RT-PCR experiments show that Rag2 is not affected, but that the targeted locus shows transcription from an alternative promoter with an alternative first exon 1b. (J:254587)