Breast cancer is the most common cancer in women worldwide and is classified into molecular subtypes based on expression of the estrogen receptor (ESR1), progesterone receptor (PGR) and HER2 (ERBB2), including luminal A, luminal B, HER2-enriched and triple negative/basal-like tumors. Key pathways in breast cancer include estrogen receptor signaling, ERBB/HER2 and EGFR signaling, PI3K-AKT-mTOR, cell cycle regulation through cyclin D1 and CDK4/6, and DNA repair genes such as BRCA1 and BRCA2. Gene expression profiling by real time PCR is widely used to study these pathways and to characterize tumor subtypes.