(a) Primary colors for genomic DNA labeling. Two MS2 (top left), two PP7 (top middle) or two boxB (top right) elements were inserted into a human telomere-specific sgRNA to generate primary colors. Shown beneath each sgRNA is live cell labeling of telomeres in human U2OS cells following co-expression of dCas9, the indicated sgRNA, and the cognate RNA binding partners fused with different fluorescent proteins. (The overlay images are on the live cell phase-contrast micrographs in this and all other image figures in this paper.) Scale bar, 5 μm. (b) Secondary colors. MS2 and PP7 (top left), PP7 and boxB, (middle left) or boxB and MS2 (bottom left) were inserted into the sgRNA so as to generate cyan, yellow or magenta, respectively, by spectral overlapping of signals from a pair of cognate RNA binding partners fused with different fluorescent proteins. Images at the right of each secondary color design are the telomere labeling images obtained after co-expression of dCas9, the indicated sgRNA, and the cognate pair of fluorescent proteins, Scale bar, 5 μm. (c) A tertiary 'color'. boxB, MS2, and PP7 were inserted into the sgRNA to generate white (left). Images at the right are telomere labeling following co-expression of dCas9, the triple element-bearing sgRNA, and the three cognate partners with distinct fluorescent proteins. Scale bar, 5 μm. Data in all panels are representative of experiments performed at least three times.