An EGFP tag has been introduced into the endogenous EcR locus, at the C-terminal end of the coding sequence. In addition, a pair of FRT sites has been inserted so that they flank the last 4 exons (which are common to all isoforms), resulting in a conditional null allele in which EcR function can be disrupted using FLP recombinase to excise this artificial FRT cassette.