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Three scenarios for the origin of isolated CRISPR arrays. CRISPR arrays are shown with dark gray rectangles (repeats) and diamonds (spacers). Flanking non-cas genes are shown with light gray block arrows, transposable elements are shown with orange blocks, and cas genes are shown with green blocks. (a) de novo formation of a CRISPR array; (b) formation of a CRISPR array mediated by transposon insertion; (c) excision of cas genes leaving an isolated array.
