A single loxP site was introduced 5' to exon 2 and a loxP-flanked thymidine kinase-neomycin fusion gene was inserted 3' to exon 3. Cre-mediated deletion of the intervening sequences was accomplished by mating mutant mice to CMV-cre mice, deleting the DNA encoding the major transactivation domain. RT-PCR analysis on spleen and thymus RNA indicated that no detectable transcript is expressed from this allele.