This is a multicenter, non-randomized, prospective cohort study. The purpose of the study is to identify constitutional genetic factors associated with histological response, resistance or sensibility to treatment in human epidermal growth factor receptor 2 (HER2)-positive breast cancer. 9000 patients will be enrolled in this study. Blood samples will be collected after informed consent and inclusion in the study. Patients will be treated and followed according to the standards of their treating center. They will be followed every six months for five years.