Ataxia Telangiectasia Mutated (ATM) kinase is a central regulator of the DNA damage response and its loss of function leads to the development of Ataxia Telangiectasia, a rare genetic disorder characterized by several features among which cerebellar neuron degeneration which causes ataxia, telangiectasia, immune system abnormalities, higher predisposition to lymphoma and leukaemia development and type 2 diabetes. The gene discussed is ATM; the disease is ataxia telangiectasia.