Two FRT sites have been inserted into the endogenous ERR locus; the first is inserted in a 5' intron, and the second is downstream of the 3' end of the gene. A Disc\RFPDsRed.3xP3.cUa marker has also been inserted, downstream of the 3' end of the ERR locus, but upstream of the second FRT site. This results in a conditional allele, in which the majority of the ERR coding sequence can be excised by using FLP recombinase.