The key cell cycle checkpoint between G2 and M phase was investigated in a retrospective cohort of breast tumour samples from 10 ER+(and/or)PR+/HER2+; 32 ER+(and/or)PR+/HER2−; 1 ER−PR−/HER2+ and 4 ER−PR−/HER2− patients. The gene discussed is ERBB2; the disease is breast neoplasm.