Construct designed for use in a single-strand annealing (SSA) assay. Contains two copies of a mini-w gene in tandem, with an I-SceI site in between the two copies. The copy downstream (3') of the I-SceI site is functional (contains a fully functional w gene expressed under the control of a basal Hsp70Bb promoter). The upstream (5') copy is non-functional, containing 500bp of homology to the downstream functional copy (homology is to the 3' part of the functional mini-w copy).