A 2A-GAL4 cassette followed by a Disc\RFP3xP3.cUa marker has been inserted into the endogenous Oamb locus, at the 3' end of the open reading frame of the RB isoform. This results in the GAL4 driver being expressed as a separate protein under the control of endogenous Oamb regulatory sequences (the expression of only the RB isoform is expected to be trapped by GAL4).