A switchable allele that allows conditional expression of a mutant form of tkv has been engineered at the endogenous tkv locus. The switchable allele contains a tandem duplication of tkv coding sequence. The 5' coding sequence is flanked by FRT sites, encodes wild-type tkv and is tagged with GFP. The 3' coding sequence is tagged with mCherry and encodes a mutated form of tkv, carrying a R409C point mutation (this change is equivalent to a R443C change in the orthologous human BMPR1A gene, a variant identified in patients with Juvenile polyposis syndrome). A single loxP site is present downstream of the 3' coding sequence. In the absence of FLP recombinase, only the 5' coding sequence (wild-type 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 mutated tkv ::mCherry coding sequence.