A TI{T-STEP.TagRFP-T} cassette has been knocked in to the endogenous Ocrl locus, resulting in wild-type Ocrl tagged at the C-terminal end with TagRFP-T. This tag can be converted to EGFP in a tissue-specific manner by using RSR recombinase to excise the RSRT cassette that includes the TagRFP-T tag.