An approximately 12kb genomic fragment containing the wild-type crb locus has been knocked-in to the crbGX24w- founder knockout line that contains an attP site (target site for phiC31:int integrase). A single attP(Bxb1) site (target site for the Bxb1:int integrase), and a loxP cassette containing a w+* marker are also present just downstream of the crb gene as a consequence of the recombination event.