WRN and Werner syndrome: We have identified the Drosophila orthologue of human WRN exonuclease (Cox et al. 2007), hypomorphic mutation of which results in WS-like phenotypes including hypersensitivity to CPT and extremely high rates of recombination (Saunders et al. 2008), and shown that the protein does indeed possess exonuclease activity (Boubriak et al. 2009).