The br disc enhancer is fused upstream of an FRT cassette that contains the tdTomato coding sequence followed by stop sequences. Sequence encoding a myristoylated form of GFP is present downstream of the FRT cassette. In the absence of FLP tdTomato is expressed under the control of the br disc enhancer. If the FRT cassette is excised, myristoylated GFP is expressed under the control of the br disc enhancer.