Transgene designed to assay the activity of the I-SceI endonuclease. Contains a non-functional 9kb HindIII w fragment (two-thirds of the first exon and 5kb of 3' untranslated region, modified to introduce a frameshift and three new restriction sites for EcoRI, NruI and XhoI) and the y+mDint2 marker. These sequences are flanked by a pair of RS(I-SceI) sites, which are in turn flanked by a pair of FRT sites.