80 HER2-positive patients treated with 1 year of trastuzumab….Better disease-free survival (DFS) was observed in patients with PIK3CA wild-type compared with mutated tumours (P=0.0063). By combining PIK3CA status and treatment arms, four separate prognostic groups with significantly different DFS (P=0.0013) were identified....Overall survival curves also differed significantly in the overall population (P=0.035) and in the treatment-based subgroups (P=0.028) in favour of PIK3CA wild-type tumours....A favourable survival benefit was observed when neoadjuvant trastuzumab was added early to neoadjuvant chemotherapy, particularly in patients with PIK3CA wild-type tumours (Supplementary Figure S1).