WRN and Werner syndrome: Werner syndrome cell lines are sensitive to the topoisomerase I poison CPT (Poot et al., 1999; Pichierri et al., 2000a) which causes replication fork collapse at the bound topoisomerase (Shao et al., 1999); such sensitivity can be partially complemented by expression of a bacterial Holliday junction nuclease (Rodriguez-Lopez et al., 2007), suggesting that WRN acts either to prevent accumulation of Holliday junctions at collapsed forks or to ensure rapid Holliday junction resolution.