a, In Brainbow-1.0, incompatible sets of lox sites alternate: Cre chooses between excision events 1 or 2. Before Cre action, only the gene following the promoter is expressed (RFP). Recombination switches expression to either YFP (1) or M-CFP (2). b, HEK cells stably transfected with CMV-Brainbow-1.0 express RFP. On transient transfection with Cre, these cells randomly switch to YFP or M-CFP expression. c, In Brainbow-1.1, a third set of incompatible lox sites (loxN) is added, creating three recombination possibilities (1, 2 or 3), switching OFP expression to RFP, YFP or CFP expression. d, Cells stably transfected with Brainbow-1.1 express OFP. Cre recombination leads to expression of M-RFP, M-YFP or M-CFP. pA, polyadenylation signal; M-XFP, membrane-tethered XFP. FRT site allows reduction of transgene arrays (Fig. 4d). Scale bar, 50 μm.