A summary of the findings presented in this paper. The ATR kinase regulates intracellular H2S concentrations (arrow 1) and the levels of the three H2S-synthesizing enzymes (arrow 2). Increased in intracellular H2S concentrations decreased ATR serine 435 phosphorylation, while decreased in intracellular H2S concentrations increased this phosphorylation (arrow 3). Attenuation of intracellular H2S synthesis also potentiates CHK1 serine 345 phosphorylation following ATR cell exposure to low t-BOOH concentration, an event not seen in ATR-H cells (arrow 4). Lastly, low cellular H2S concentrations in the hypomorphic ATR-H cells increase genomic instability by itself, and when combined with a low dose of t-BOOH. Taken together, our data suggests that the ATR kinase regulates and is in turn regulated by H2S.