PTEN and cancer: The phosphoinositide 3-kinase (PI3K)/AKT/phosphatase and tensin homologue (PTEN) pathway, which promotes cell proliferation and resistance to apoptosis, is activated in several different types of cancer.3, 4 In ER-positive, HER2-negative breast cancers, PI3K/AKT/PTEN pathway activation occurs most frequently through PIK3CA mutations that activate the catalytic p110α subunit of PI3K.5, 6, 7, 8 Approximately 40% of advanced ER-positive tumours carry activating PIK3CA mutations.