The Phase II METTEN trial suggested that the C allele of ataxia telangiectasia mutated (ATM) rs11212617 might function as a predictive biomarker to guide the personalized use of metformin in patients with breast cancer (Cuyàs et al., 2019a); however, subsequent findings from the Phase III MA.32 trial refuted this notion (Goodwin et al., 2021b). This evidence concerns the gene ATM and breast carcinoma.