The exon encoding mature dpp has been replaced by sequence encoding: 1. an artificial FRT cassette that contains a splice acceptor followed by the exon encoding mature dpp, tagged with Tag:HA such that the mature protein is tagged, 2: a splice acceptor followed by the dpp pro-domain tagged with Tag:M(mCd8a) and GFP. In the absence of FLP, functional dpp is produced from the first part of the inserted sequence. After excision of the FRT cassette using FLP recombinase, the Tag:M(mCd8a)-GFP reporter is expressed under the control of endogenous dpp regulatory sequences.