A targeting vector was designed to insert an "IRES-EGFP-floxed NEO" cassette (containing an internal ribosomal entry site (IRES), enhanced green fluorescent protein (EGFP) sequence, and a loxP-flanked neo) between the stop codon and endogenous polyA signal of the targeted gene. This construct was electroporated into (C57BL/6 x 129S4Sv/Jae)F1-derived V6.5 embryonic stem (ES) cells. Correctly targeted ES cells were transiently transfected with a cre expressing plasmid to remove the neo selection cassette. (J:130248)