UAS regulatory sequences drive expression of a phosphorylation substrate region (in this case from the mammalian CrkII protein) that is sandwiched between ECFP and EYFP and that has been mutated to carry the Y221F amino acid substitution; this mutation renders the natural Abl phosphorylation site unphosphorylatable. The P{UAS-Abl.Y221F.FRET} transgene acts as a negative control for experiments that use the P{UAS-AblFRET} Abl activity sensor; the mutation in P{UAS-Abl.Y221F.FRET} abolishes changes in FRET in response to Abl activity (PMID:11752449).