Key biomarkers every breast cancer patient should know — what they mean, why they matter, and how they guide treatment decisions.
| Biomarker | What It Tells You | Type | Treatment Implications |
|---|---|---|---|
| ER (Estrogen Receptor) | Whether cancer cells grow in response to estrogen | Predictive | ER+ tumors respond to endocrine therapy (tamoxifen, aromatase inhibitors) |
| PR (Progesterone Receptor) | Hormone sensitivity; often tested alongside ER | Predictive | PR+ supports endocrine therapy benefit; PR- may indicate poorer prognosis |
| HER2 | Overexpression of growth-promoting protein | Therapeutic | HER2+ patients benefit from trastuzumab (Herceptin), pertuzumab, T-DXd, tucatinib |
| Ki-67 | How fast cancer cells are dividing | Prognostic | High Ki-67 (>20%) suggests aggressive tumor; may indicate chemo benefit |
| BRCA1 / BRCA2 | Inherited DNA repair gene mutations | Therapeutic | PARP inhibitors (olaparib, talazoparib); platinum-based chemo; family screening |
| PIK3CA | Somatic mutation in PI3K signaling pathway | Therapeutic | Alpelisib (Piqray) for HR+/HER2- with PIK3CA mutation |
| PD-L1 | Immune checkpoint protein expression | Predictive | PD-L1+ triple-negative: pembrolizumab (Keytruda) + chemo |
| Oncotype DX | 21-gene recurrence score (0–100) | Prognostic | Low score (≤25 in most): endocrine therapy alone; high score: add chemo |
| MammaPrint | 70-gene signature classifying genomic risk | Prognostic | Low-risk: may safely omit chemotherapy; high-risk: chemo recommended |
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