A YFP/icre fusion construct with an upstream IRES element and a downstream frt-flanked neomycin resistance cassette was placed into the downstream untranslated region of the gene. Chimeric mice were crossed with FLPe expressing mice to remove the neomycin selection cassette. YFP-cre expression based on YFP flouresecene was only detected in regulatory T cells. Crossing these mice with mice that contain a cre-mediated reporter gene also demonstrated low level cre expression in about 2-10% of hematopoietic lineage cells including immature and mature B cells, T cells, myeloid cells and bone marrow precursor cells. (J:134513)