ATR and Werner syndrome: However, both WS and ATR-SS fibroblasts show telomere instability (Ariyoshi et al. 2007; Crabbe et al. 2004; Pirzio et al. 2008; Casper et al. 2004, 2002), which may explain the observed increased frequency of telomere-derived sister chromatin exchanges (T-SCEs) seen in these syndromes (McNees et al. 2010; Hagelstrom et al. 2010).