A, If two sites of a SSR are in relative proximity on the DNA in opposite orientations, SSR-mediated recombination between these two sites results in an inversion of the DNA fragment between the two sites (upper panel). If the two sites are in the same orientation, SSR-mediated recombination between these two sites results in an excision of the DNA fragment between the two sites, and the appearance of a circular DNA corresponding to this excised fragment (lower panel). B, SSRs frequently used in gene targeting, and their corresponding sites.