Most breast cancers are hormone receptor positive and exhibit a slow growth pattern. Based on biological properties, breast cancers are divided into four different biological subtypes. Furthermore, these subtypes are indicative of the risk of recurrence, which is also influenced by the size of the tumor and extension to lymph nodes. Postoperative adjuvant drug therapy is chosen on the basis of the biological type. Chemotherapy can be used in all subtypes. Hormonal therapies are used exclusively for the treatment of hormone receptor positive breast cancer. Trastuzumab antibody belongs to the treatment of the HER2 positive subtype.