Proposed mechanisms of casposon replication and integration. (a) Mechanism of casposon DNA excision and replication. (b) Mechanism of spacer acquisition by CRISPR-Cas system. Adapted from . (c) Proposed mechanism of Cas1-mediated casposon integration. See text for details. Subterminal regions within TIRs outlined with the broken line indicate that processing occurs only in a fraction of casposons. See text for details. Abbreviations: TIR, terminal inverted repeats; TSD, target site duplication.