Abstract
Breast cancer diagnosis and treatment increasingly rely on biomarker-driven strategies to improve patient stratification and therapeutic precision. Conventional biomarkers, including estrogen receptor (ER), progesterone receptor, human epidermal growth factor receptor 2 (HER2), and the proliferation marker Ki-67, remain central to routine clinical decision-making. Over the past 5 years, substantial advancements have refined the assessment, interpretation, and clinical application of these established markers, while additional biomarkers have transitioned into standard practice. Updated international guidelines have introduced refined classifications such as ER-low tumors and improved HER2 testing algorithms, reducing equivocal results and enhancing diagnostic accuracy. Standardization initiatives led by international working groups have improved the reproducibility of Ki-67 scoring, supporting its prognostic and, in selected settings, predictive utility. In parallel, several newly integrated biomarkers have expanded the diagnostic and therapeutic landscape of breast cancer. Programmed death-ligand 1 expression is now routinely assessed in advanced triple-negative breast cancer to guide immune checkpoint inhibitor therapy. Genomic alterations, including PIK3CA mutations and germline BRCA1/2 variants, have become essential biomarkers for selecting patients eligible for targeted therapies such as PI3K and PARP inhibitors. More recently, detection of ESR1 mutations has enabled personalized endocrine therapy in metastatic hormone receptor-positive disease. Collectively, these developments reflect a shift toward precision oncology, integrating conventional pathology with molecular and immune profiling. This mini-review highlights clinically approved and guideline-endorsed biomarkers introduced or refined between 2018 and 2023, emphasizing their impact on diagnostic accuracy, therapeutic selection, and patient outcomes. Continued refinement and harmonization of biomarker assessment will further strengthen individualized management strategies in breast cancer.