Two B3RT sites have been inserted into the endogenous Tdc2 locus, along with sequence encoding a lexA::UnkAD driver downstream of the 3' B3RT site. In the presence of B3R recombinase Tdc2 coding exons are excised, resulting in the lexA::UnkAD driver being expressed under the control of endogenous Tdc2 regulatory sequences. Tdc2 function is affected before and after excision.