A sequence cassette encoding 1. the coding sequence of GFP, tagged at the N-terminal end with the Tag:SS(wg) signal peptide, and 2. a w+* marker, has been inserted into the attP site in TI{TI}ptcKO.attP (insertion in the 5'UTR of the endogenous ptc locus). This results in expression of secreted GFP under the control of endogenous ptc regulatory sequences. The loxP cassette containing a Disc\RFP3xP3.cUa marker that was present in TI{TI}ptcKO.attP is still present, downstream of the inserted GFP sequence.