https://www.selleckchem.com/products/cx-5461.html Breast cancer is a heterogeneous disease with well-known characteristics such as hormone receptor (HR) status and human epidermal growth factor (Her)2 status. Although Her2 represents an established treatment target, the development of resistance mechanisms during treatment, cardiotoxicity, and a worse response to standard therapies lead to worse outcomes. Therefore, we investigated various biomarkers in breast cancer such as Her2 mutations, Her2 heterogeneity, HR, PIK3CA, PTEN, programmed death receptor ligand 1 (PD-L1), tumor-infiltrating lymphocytes (TIL), micro RNA (miRNA), and BRCA mutations with regard to their clinical impact in Her2-positive disease. HR status and Her2 status, such as the presence of PIK3CA mutations, already play a role in treatment decision-making processes, whereas other biomarkers like PD-L1 status or TIL represent promising future markers. The influence of BRCA mutations in Her2-positive disease, Her2 mutations, and the impact of miRNA is vague to date. Antibody-drug conjugates (ADC) such as T-DM have been established as important treatment strategies, especially in Her2-positive disease. However, up-to-date biomarkers appropriate for clinical practice are missing. Further studies are needed. However, up-to-date biomarkers appropriate for clinical practice are missing. Further studies are needed.Currently, the dichotomous definition of human epidermal growth factor receptor 2 (HER2)-positive versus HER2-negative disease undergoing a change through inclusion of the identification of the "HER2-low" category, for which new therapeutic compounds in the form of potent antibody drug conjugates (ADC) may be effective. In addition, resistance to HER2-directed targets has become a clinical challenge and, therefore, strategies to bypass the HER2 receptor are of high interest. These are new HER2 ADCs and tyrosine kinase inhibitors, such as tucatinib or neratinib. The underlying mechanisms of resist