The M{DR-white} construct is designed to measure the repair of an induced double strand break (DSB) in vivo; it directly measure the frequency of homologous recombination (HR) and also detects nonhomologous end-joining (NHEJ) and single-strand annealing (SSA) events. It contains two non-functional copies of the w gene; 'Sce.white' and 'iwhite', 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. 'iwhite' 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. 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. Individual premeiotic germline events can be analyzed by mating males containing both M{DR-white} and a source of I-SceI to y[-] w[-] females. Three different phenotypic outcomes are possible in the progeny, depending on the type of DSB repair; 1. y[+] w[+] animals due to non-crossover intrachromosomal HR (gene conversion of the RS(I-SceI) sequence to the wild-type SacI sequence has occurred), 2. y[+] w[-] animals due either to intersister HR, NHEJ without processing, no DSB, or NHEJ with processing, 3. y[-] w[-] animals due to SSA.