A loxP site and the human RET (RET9) sequence was inserted with a floxEGFPfrt-neo-frt cassette. The neo was removed by flp mediated recombination. This allele allows EGFP expression upon the cre mediated deletion of floxed cDNA from exon 1. Cre-mediated recombination removed the cDNA and allows for the expression of the reporter gene.