This study aims to investigate the link between the Ataxia Telangiectasia Mutated (ATM) gene and metformin response. This link has been identified from large studies of the human genome, and this study aims to confirm this link in a clinical study. The ATM gene is involved in DNA repair - if a person inherits a "faulty" copy of this gene from both their parents, they have a genetic condition called Ataxia-telangiectasia (A-T).