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A hypothetical scenario for the origins of type IV CRISPR-Cas systems. Homologous genes are shown as color-coded arrows or rectangles (for domains) and identified by a family name. The key evolutionary events are described to the right of the images. Optional genes and CRISPR arrays are denoted by dashed outlines. GGDD: key catalytic motif of the cyclase/polymerase domain of Cas10.
