 Study population will consist of two separate cohorts of patients with HER2-positive breast cancer that will be analysed based on their responsiveness to trastuzumab-containing chemotherapy. Cohort 1 will include patients with TNM stage II-IV breast cancer with highly trastuzumab-sensitive tumours. Results of laboratory analyses from Cohort 1 will be matched with those from Cohort 2 Control group that includes patients at the same TNM clinical stages but with trastuzumab-refractory disease. 