A DNA cassette comprising 1. two open reading frames (EGFP and GAL4) separated by a T2A sequence, 2. a loxP cassette containing the Disc\RFP3xP3.PB marker has been inserted between the promoter and coding sequence of the endogenous piwi locus. The EGFP sequence is non-functional. The insertion results in GAL4 being expressed under the control of the endogenous piwi regulatory sequences, instead of the piwi coding sequence.