Serine/threonine-protein kinase ATR (ATR) catalyzes the addition of a phosphorate to serine or threonine residues. The protein plays a central role in cell-cycle regulation at the G2 phase checkpoint in response to single stranded DNA, DNA damage, and/or a replication block, but is not required for G2-arrest. This protein functions by transmitting DNA damage signals to downstream effectors of cell-cycle progression. Research has suggested that ATR may phosphorylate histone variant H2AX to form H2AXS139ph at sites of DNA damage to regulate the DNA damage response mechanism.

ATR is also required for the basal expression of RNR1 (ribonucleotide reductase large subunit) and for effective immune responses that involve the activation of DNA damage responses, as well as acting with telomerase to maintain telomeric DNA tracts. However, ATR is not required for telomere length homeostasis.