In conclusion, this is the first clinical study with longitudinal faecal sampling in breast cancer patients that explored the associations between adriamycin, cyclophosphamide and docetaxel treatment and the intestinal microbiota, as well as the impact of the intestinal microbiota on chemotherapy toxicity and tumour response in ER+ and HER2− postmenopausal breast cancer patients. Here, ERBB2 is linked to neoplasm.