The premature ageing syndrome protein, WRN, is a 3′→5′ exonuclease.

Tests the prediction that WRN, the gene defective in Werner syndrome, is an exonuclease. Use of tagged recombinant wild-type and mutant WRN proteins; Finding that WRN is indeed an endonuclease, which resides in the N terminus; Difference from other human RecQ-like helicases; Possible explanation for the differences between Werner syndrome and Bloom's syndrome.