The activation of DNA-damage checkpoints enforces growth arrest of damaged cells to allow DNA-repair mechanisms to repair the damaged DNA. Altered pH regulation and Na+/H+ exchanger 1 (NHE1, SLC9A1) activity are characteristic for cell division and cell death 1,2. Checkpoint recovery constitutes a specialized form of mitotic entry and several lines of evidence suggest that this may be pH-dependent. Further, a global profile of proteins phosphorylated by Ataxia telangiectasia mutated (ATM) after UV-induced DNA damage includes NHE1 3. However, the possible link between pH-regulation in DNA damage response is incompletely understood. By bioinformatic screening, we found that the distal C-terminus of NHE1 (NHE1dCT) contains a consensus motif [784DSPSSQR790, human sequence] for phosphorylation by ATM. Here we investigate: (i) whether NHE1 is a substrate for ATM in vitro, using human (h)NHE1 constructs, and (ii) the roles of NHE1 activity and pH regulation in DNA-damage response and cell death in fibroblasts expressing wild type (WT) rat (r)NHE1 (PS120 cells) and rat NHE1-S793A (corresponding to S788 in hNHE1) (PS120-S793A cells). By using an in vitro kinase assay we found, that the hNHE1-637-815 distal C-terminal tail (dCT) is phosphorylated by ATM in vitro in a manner fully dependent on Ser788 (n=3). We further tested the influence of the specific ATM phosphorylation site on NHE1 activity and pH regulation. To induce a DNA-damage response and activation of ATM, the cells were treated with (10 μM) of the topoisomerase II inhibitor etoposide for 24 h (n=6) or the antibiotic zeocin for 6 h (n=7). Both conditions induced a significant increase in the rate of intracellular pH (pHi) recovery after an NH4Cl-prepulse-induced acid load in PS120 cells expressing wild type WT rNHE1. In contrast, no increase in pHi recovery rate was seen in cells expressing rNHE1-S793A that cannot be phosphorylationed by ATM. Additionally, inhibiting NHE1 with amiloride analogs 5-(N-ethyl-N-isopropyl) (EIPA) (10 µM) sensitized PS120-S793A cells to etoposide-induced loss of cell viability as assessed by MTT assay, whereas it enhanced viability after etoposide treatment in PS120 cells (n=3). In conclusion, we identified a specific ATM phosphorylation site at Ser788 at the distal C-terminus of hNHE1 and showed that NHE1 activity is increased during DNA damage. Analyses to address the significance of NHE1 regulation by ATM in the DNA damage response and checkpoints are ongoing.