A targeting vector was designed to insert an internal ribosome entry site (IRES) fused to a CreER fusion gene followed by a frt-flanked neo cassette upstream of the stop codon of the Tcerg1l (transcription elongation regulator 1-like) gene. CreER incorporates cre recombinase fused to a G400V/M543A/L544A triple mutation of the human estrogen receptor ligand-binding domain. The neomycin cassette was excised with Flp recombinase.