ATM is a 370 kDa nuclear phosphoprotein involved in Ataxia Telangiectasia. It is a serine/threonine protein kinase which activates checkpoint signaling, apoptosis and genotoxic stresses, thereby acting as a DNA damage sensor. It is found in pancreas, kidney, skeletal muscle, liver, lung, placenta, brain, heart, spleen, thymus, testis, ovary, small intestine, colon, and leukocytes.