Increasing evidence exists that activation of the PI3K pathway or the MAPK pathway in breast cancer is associated with a higher chance to develop resistance to anti-oestrogens or oestrogen deprivation.4,13,14 Pathway activation may also influence prognosis.8,15–17 Therefore, research has focused on the identification of predictive and prognostic biomarkers associated with pathway activation that might be relevant for patient selection. This evidence concerns the gene PIK3CA and breast carcinoma.