Repair of a replication fork stalled at a DNA lesion. When a replication fork encounters a blocking lesion in the leading strand, the fork stalls. Fork regression creates a Holliday intermediate and reunites the lesion-containing strand with the complementary template strand with which it was originally paired. Lesion repair can then proceed by the usual pathways. Replication can be restarted by digestion of the short arm and reloading of the replisome (steps 2 and 3, left). Alternatively, the 3′ end of the short arm can be extended to the end of the available template (step 2, right). If the lesion is not repaired, branch migration in the direction opposite to fork regression pairs the newly synthesized strand with the lesion (step 3, right). Replication is restarted, and repair can occur later.