Integration of a T-STEP cassette into the genome using CRISPR/Cas9-based gene targeting allows tissue-specific tagging of endogenous proteins (T-STEP). The endogenous protein is tagged with a different fluorescent protein depending on whether or not the RSR recombinase has been used to remove DNA sequence between a pair of tandem RSRT sites present in the cassette. When a pT-STEP ( Addgene:72334 ) based plasmid is used as the donor vector for gene targeting, the two fluorescent protein tags are TagRFP-T and EGFP. In the absence of the RSR recombinase (inserted sequence represented as TI{T-STEP.TagRFP-T} in FlyBase), the targeted gene is tagged with the TagRFP-T (with translation of an in-frame RSRT recognition sequence acting as a short peptide linker between the the targeted protein and the tag). In the presence of the RSR recombinase (inserted sequence lacking the RSRT cassette represented as TI{T-STEP.EGFP} in FlyBase), the targeted gene is instead tagged with EGFP (again, translation of an in-frame RSRT recognition sequence acts as a short peptide linker between the the targeted protein and the tag).