The M{Sce.white} construct contains 'Sce.white', a copy of the w gene that 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. A y+t* marker is also present downstream of the 'Sce.white' sequence. The construct acts as a target for repair in an assay to measure interhomolog homologous recombination (HR). Expression of I-SceI can induce a DSB at the RS(I-SceI) site in M{Sce.white}, and gene conversion of the 'Sce.white' sequence using a M{iwhite} or M{iwhiteΔ9} donor template at the same allelic locus on the homologous chromosome can produce a functional w gene, resulting in progeny with red or orange eyes respectively.