Fibroblasts from patients with HGPS show increased levels of basal phosphorylated histone variant H2AX (γH2AX) and increased amounts of phosphorylated checkpoint kinase 1 (CHK1) and CHK2, compared with unaffected fibroblasts (Liu Y. et al., 2006). Here, CHEK1 is linked to Hutchinson-Gilford progeria syndrome.