a, In Brainbow-2.0, Cre triggers inversion of a DNA segment flanked by loxP sites in opposite orientation. In 50% of cells, inversion should end in an antisense orientation and switch gene expression. b, HEK cells stably expressing CMV-Brainbow-2.0 produce RFP, and stochastically switch to CFP expression when transfected with Cre. c, The Brainbow-2.1 construct contains two tandem invertible DNA segments. Inversion (i–iii) and excision (iv, v) recombination events create four expression possibilities. d, Stable CMV-Brainbow-2.1 transfectants express nuclear GFP (nGFP). Cre recombination triggers expression of YFP, RFP or M-CFP. pA1 and pA2, SV40 and bGH polyadenylation signals. Scale bars, 50 μm.