A switchable allele containing a tandem duplication of wild-type tkv coding sequence has been engineered at the endogenous tkv locus; this allele is used as a control. The 5' coding sequence is tagged with GFP and the 3' coding sequence is tagged with mCherry. The 5' coding sequence is flanked by FRT sites. A single loxP site is present downstream of the 3' coding sequence. In the absence of FLP recombinase, only the 5' coding sequence (tkv ::GFP) is expressed (transcription terminates prior to the second coding sequence). In the presence of FLP recombinase, the FRT cassette containing tkv ::GFP is excised, allowing expression of the tkv ::mCherry coding sequence.