The M{DR-whiteΔ9} construct is designed to measure the frequency of intrachromosomal homologous recombination (HR) repair in vivo. It contains two non-functional copies of the w gene; 'Sce.white' and 'iwhiteΔ9', with the two copies separated by a y^+t* marker. 'Sce.white' is non-functional due to the insertion of an 18bp RS(I-SceI) site (plus an additional nucleotide) at the wild-type SacI site, resulting in an in-frame premature stop codon. 'iwhiteΔ9' is non-functional due to 5' and 3' truncations that remove the promoter, 5' UTR, start codon, the last 30 amino acids, and the 3' UTR, and also contains a 9bp in-frame deletion in the coding sequence that removes a SacI site. Expression of I-SceI can induce a DSB at the RS(I-SceI) site, and this DSB can be repaired in premeiotic germline cells and mitotically dividing somatic cells. HR events can be analyzed by mating females carrying M{DR-whiteΔ9} in a y[-] w[-] background to males expressing a source of I-SceI. When the DSB is repaired by intrachromosomal HR, the 'Sce.white' sequence is converted to 'white&gr;9', a functional white gene containing the 9bp deletion from the 'iwhiteΔ9' donor template, resulting in flies with orange eyes in the F2 generation.