Probable serine/threonine kinase. Seems to play a central role in cell-cycle regulation by transmitting DNA damage signals to downstream effectors of cell-cycle progression. May recognize the substrate consensus sequence [ST]-Q and phosphorylate histone variant H2AX to form H2AXS139ph at sites of DNA damage, thereby regulating DNA damage response mechanism (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Belongs to the PI3/PI4-kinase family. ATM subfamily.