ING3 (Inhibitor of Growth 3) is a candidate tumor suppressor gene whose expression is lost in tumors. Aims: We want to identify and characterize new tumor suppresor functions for ING3. Methods: We conduct experiments in yeast and human cells depleted or not for ING3 and exposed to genotoxic agents. Results: - ING3-depleted human cells and yeast cells deleted for its ortholog YNG2 are sensitive to DNA damage suggesting a conserved role in response to such stress. - In human cells, ING3 is recruited to DNA double strand breaks and is required for ATM activation. - In response to doxorubicin, ATM activation is dependent on ING3 but not on TIP60, whose recruitment to DNA breaks also depends on ING3. - These events lead to ATM-mediated phosphorylation of NBS1 and of major mediators of the DNA damage response. - Upon genotoxic stress, DNA repair by Non Homologous End Joining (NHEJ) or Homologous Recombination (HR) were impaired in absence of ING3. - Immunoglobulin Class Switch Recombination (CSR), a physiological mechanism requiring NHEJ repair, was impaired in the absence of ING3. Conclusions: Since deregulation of DNA double strand break repair is associated with genomic instability, we propose a novel function of ING3 as a caretaker tumor suppressor involved in the DNA damage signaling and repair. Funding: A.M. was a recipient of a doctoral fellowship from La Ligue Contre le Cancer and Region Bretagne. R.P. & M.G. were recipients of from Ulysses “The France – Ireland Exchange Scheme”. N. Bigot was a recipient of ANR program (SAFE 2012) (ANR-11-RPIB-0012). C. References: Cell Death Differ. 2019 Feb 25. doi: 10.1038/s41418-019-0305-x. Epub ahead of print