However, recent data has shown that premature cellular senescence only occurs in a subset of PSs that includes WS, AT, HGPS, and ATR-SS (a version of Seckel Syndrome that has mutations in the AT and Rad3-related checkpoint kinase ATR) (Tivey et al. 2013a, 2013b; Davis et al. 2013b). The gene discussed is ATR; the disease is microcephalic primordial dwarfism.