Using MRS, we and others have previously reported alterations in the levels of choline metabolites and/or lactate in response to different PI3K/mTOR pathway inhibitors in vitro and in vivo in various cancer models.18–28 However to the best of our knowledge, metabolic biomarkers for AKT inhibitors have only been evaluated in vitro and ex vivo in breast cancer models,26,27,29 and there are no previous metabolic biomarker studies in vivo in tumour xenografts. The gene discussed is PIK3CA; the disease is breast cancer.