ATM/ATR-Dependent DNA Double-Strand Break Repair Response Is Activated During Repair of High NaCl-Induced DNA Breaks. Common DNA damage repair responses include transient cell cycle arrest, during which the DNA repair occurs. We tested to see whether that happens during the disappearance of the DNA breaks after reduction of high NaCl (Fig. 2). Indeed, phosphorylation of checkpoint kinase 1 (Chk1), which contributes to all defined cell cycle checkpoints (21), occurs rapidly, and the phosphorylation is reduced by caffeine, which is an ATM/ATR inhibitor (22) (Fig. 2A). Similarly, G2/M cell cycle arrest activates rapidly when NaCl decreases, and this arrest is abrogated by caffeine (Fig. 2B). Repair of the breaks after NaCl is reduced is accompanied by formation of foci containing phosphorylated H2AX (γH2AX) (1, 5). This histone modification occurs around DNA double-strand breaks and contributes to their repair (13, 14). γH2AX is induced to a maximal level within 15 min after lowering NaCl, then gradually decreases (Fig. 2C) accompanying repair of the DSBs (Fig. 1C). The γH2AX induction is also sensitive to inhibition of ATM/ATR by caffeine (Fig. 2C). These results indicate that, when elevated NaCl is lowered, a classical ATM/ATR-dependent DNA damage response becomes activated, and they further confirm that DNA DSBs increase upon exposure to high NaCl.