The M{DR-white.mu} 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 can also be used to study gene conversion tract length and directionality in cases of repair by HR. It contains two non-functional copies of the w gene; 'Sce.white' and 'iwhite.mu', 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.mu' 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. In addition, 28 silent polymorphisms have been introduced along the length of the iwhite.mu sequence. 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.mu} 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. The polymorphisms present in the iwhite.mu sequence mean that in cases of repair by HR, gene conversion tract direction and length can be analyzed at high resolution.