ATR and Werner syndrome: In WS or ATR-SS cells the lack (or low levels) of WRNp and ATR results in a high level of replication stress (Ammazzalorso et al. 2010; McNees et al. 2010; Ozeri-Galai et al. 2008; Pirzio et al. 2008) due to a high degree of stalling at DNA replication forks (Rodriguez-Lopez et al. 2002; Koundrioukoff et al. 2013), particularly at hard-to-replicate sequences such as common fragile sites (CFS), and these sites are expressed in both WS and ATR-SS cells (Casper et al. 2004; Pirzio et al. 2008).