1.7 kb of the locus containing exons 1, 2 and part of 3 was fused to an IRES-cre construct followed by a FRT-flanked neomycin selection element. After targeted recombination, the neomycin element was removed by crossing founder mice with FLPe transgenic mice. Insertion of the cre expression construct disrupted the endogenous locus, creating a null allele. Cre expression was detected in male but not female germline cells, in memory and regulatory CD4+ T cells, and in some CD8+ and naive CD4+ T cells.