HER2- refers to a subtype of breast cancer that does not overexpress the HER2 protein, which is involved in cell growth and division. In these cases, the cancer cells lack the high levels of HER2 receptors, making them different from HER2+ breast cancers, which can be more aggressive and may respond to targeted therapies. Patients with HER2- breast cancer typically have a better prognosis and may be treated with hormone therapies or chemotherapy, depending on other factors. Understanding the HER2 status is crucial for determining the most effective treatment plan for individuals diagnosed with breast cancer.