WRN and Werner syndrome: These agents lead to replication fork arrest or collapse, suggesting a function for WRN in DNA replication, which is further supported by its presence at replication foci coincident with RP-A and PCNA (Constantinou et al., 2000; Rodriguez-Lopez et al., 2003) and aberrant replication fork progression in WS fibroblasts (Rodriguez-Lopez et al., 2002).