The P{UAS-Brainbow.2.1R} transgene allows stochastic expression of one of multiple possible fluorescent proteins from a single transgene using P1cre mediated recombination. It contains the 'Brainbow-2.1R' sequence (PMID:17972876) expressed under the control of UASt regulatory sequences. The 'Brainbow-2.1R' sequence consists of two invertible loxP cassettes, with each cassette containing open reading frames (ORFs) for two fluorescent proteins. The first cassette contains Tag:NLS(Unk)-tagged hrGFPII in the forward orientation and EYFP in the reverse orientation, and the second cassette contains tdimer2(12) in the forward orientation and Tag:MyrPalm(mLyn)-tagged mCer in the reverse orientation. Each ORF is followed by a termination sequence. In the absence of P1cre recombinase the ORF adjacent to the UASt sequence, Tag:NLS(Unk)-tagged hrGFPII, is expressed. When P1cre is present, any one of the four fluorescent ORFs can be expressed, depending on which inversion and/or excision events take place.