METTL3 depletion promotes the proliferation and migration and decreases the drug sensitivity of HR+HER2− BC via regulation of the CDKN1A/EMT and m6A-BAX/caspase-9/-3/-8 signalling pathways, which suggests METTL3 played a tumour-suppressor role and it could be a potential biomarker for predicting the prognosis of patients with HR+HER2− BC. The gene discussed is ERBB2; the disease is neoplasm.