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 to DNA lesions in order to initiate the DNA repair by homologous recombination. Phosphorylates histone H2A to form H2AS128ph (gamma-H2A) at sites of DNA damage, also involved in the regulation of DNA damage response mechanism. Required for cell growth and meiotic recombination (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Localizes to nuclear DNA repair foci in response to DNA double strand breaks. Belongs to the PI3/PI4-kinase family. ATM subfamily.