HER2/neu is a gene that provides instructions for making a protein involved in cell growth and division. In some cancers, particularly breast cancer, this gene can be overexpressed, leading to uncontrolled cell growth. This overexpression is associated with a more aggressive form of the disease. Testing for HER2/neu status is crucial in cancer diagnosis and treatment planning. If a tumor is found to be HER2-positive, targeted therapies, such as trastuzumab (Herceptin), can be used to improve treatment outcomes. Understanding HER2/neu helps doctors tailor therapies to individual patients.