A BAC containing the Rorc gene and 70Kb of upstream sequence was used to generate a transgene vector. A reporter construct encoding EGFP was knocked into the ATG start codon for the Rorc(gamma-t) isoform of the gene locus. An estimated 5-10 copies of the BAC transgene were integrated, as assessed by quantitative PCR. In mice bearing this transgene, EGFP expression was tightly correlated to Rorc(gamma-t) expression including in fetal lymphoid tissue inducer cells, double positive thymoyctes, and intestinal T cells.