ERBB2 and breast cancer: KEYNOTE-028 is an ongoing phase 1b study evaluating the efficacy and safety of pembrolizumab in patients with PD-L1 positive (expression in stroma or ≥1% tumour cells) advanced solid tumours.36 The breast cancer cohort comprises 25 patients with ER+/HER2− tumours, a breast cancer subtype generally considered to be less immunogenic than TNBC or HER2+ disease.