The targeting vector was designed with a CMV enhancer/chicken beta-actin core promoter (pCA) driving expression of a loxP-flanked, N-terminal membrane-tagged, optimized DsRed fluorescent protein variant (called tandem-dimer-Tomato or tdTomato) sequence followed by a polyadenylation signal. Immediately distal to the second loxP site is an N-terminal membrane-tagged, enhanced green fluorescent protein (EGFP) sequence itself followed by a polyadenylation signal. An frt-flanked neo cassette was also located distal to the expression vector. Red fluorescence is detected in all tissues tested. When mice are bred to Cre expressing mice, the floxed region is excised in Cre-expressing tissue and this allows expression of the EGFP cassette.