Alternative mechanisms for DSB-mediated intron homing. Subsequent to cleavage by the homing endonuclease (a), the recipient, intronless allele (thick lines) undergoes exonucleolytic degradation and homologous sequence alignment with an intron-containing donor (thin lines) (b, c). A 3' end of the recipient invades the donor, which serves as a template for repair synthesis (d). In the DSBR pathway (left), DNA synthesis through the intron (red) results in formation and expansion of a D-loop (e), which then serves as substrate for repair synthesis of the noninvading strand (f). Holliday junctions are resolved to produce either noncrossover (h) or crossover (i) products. During synthesis-dependent strand annealing (SDSA) (right), the displaced loop or bubble migrates with the replicative end as DNA synthesis proceeds through the intron (e' - g'). The newly synthesized strand is released from the donor and serves as template for repair synthesis of the noninvading strand (g' - h') to generate noncrossover products only (h'). Functions implicated in homing and their putative association with appropriate steps in the homing pathways are shown.