Serine/threonine protein kinase which activates checkpoint signaling upon genotoxic stresses such as ionizing radiation (IR), ultraviolet light (UV), or DNA replication stalling, thereby acting as a DNA damage sensor. Recognizes the substrate consensus sequence [ST]-Q. Recruited by the MRX-complex to sites of DNA lesions immediately after damage to initiate non-homologous end-joining (NHEJ). Subsequently displaced by the RPA complex in a reaction probably involving the SAE2 protein. Phosphorylates MRE11 and XRS2, 2 subunits of the MRX-complex. The phosphorylation of MRE11 is a feedback response from the checkpoint signaling pathway. Phosphorylates RAD9, CHK1 and RAD53, leading to the activation of the CHK1 and RAD23 kinases involved in the DNA damage response cascade. Phosphorylates histone H2A to form H2AS128ph (gamma-H2A) at sites of DNA damage, also involved in the regulation of DNA damage response mechanism. Phosphorylates also SLX4 and RTT107 which are involved in genome stability. Required for the control of telomere length and genome stability. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with XRS2 and associates with DNA double-strand breaks. Localizes to nuclear DNA repair foci in response to DNA double strand breaks. Belongs to the PI3/PI4-kinase family. ATM subfamily.