Ki-67

**Biology:** Ki-67 (encoded by MKI67 on chromosome 10) is a large nuclear non-histone protein associated with the perichromosomal layer during mitosis. It acts as a biological surfactant that keeps mitotic chromosomes separated and helps organize heterochromatin. It is present throughout G1, S, G2, and M phases, peaking in M phase. It is absent in G0 and early G1 after mitosis, and it degrades rapidly. Its half-life is short (about 1 hour), so staining reflects currently cycling cells.

**Detection:** Immunohistochemistry using the MIB-1 monoclonal antibody (works on formalin-fixed, paraffin-embedded tissue). Results are reported as the Ki-67 labeling (proliferation) index, the percentage of tumor cell nuclei that stain positive. Staining is nuclear.

**Clinical significance:** - **Prognostic marker:** A higher index generally indicates a more aggressive, rapidly proliferating tumor and a worse prognosis. It also often predicts better response to chemotherapy, because chemotherapy preferentially kills dividing cells. - **Neuroendocrine tumors (GEP-NETs):** Ki-67 index is part of WHO grading. G1 is <3%, G2 is 3-20%, and G3 is >20%. Poorly differentiated neuroendocrine carcinomas typically have a very high index (often >50-70%). - **Breast cancer:** Helps distinguish luminal A (low Ki-67, ER+/PR+, HER2-) from luminal B (higher Ki-67). Cutoffs vary and interlab reproducibility is limited. - **Lymphomas:** Very high in Burkitt lymphoma (~100%, "starry sky" pattern, t(8;14) with c-MYC). It is also high in other aggressive lymphomas such as DLBCL. Low in indolent lymphomas such as follicular lymphoma. It helps distinguish follicular lymphoma (low Ki-67 in neoplastic follicles) from reactive follicular hyperplasia (high Ki-67 in germinal centers, with polarized staining). - **Other uses:** CNS tumors (e.g., distinguishing meningioma grades, and glioma grading, where a higher index suggests higher grade), GISTs, melanoma, and prostate cancer. It can also help distinguish benign from malignant proliferations.

**Limitations:** It is not specific for malignancy, because normal proliferating tissues (germinal centers, basal crypt cells, basal epidermis) are also positive. There is interobserver and interlaboratory variability, and cutoffs are not universally standardized.

**Board pearls:** - Ki-67 = marker of cell proliferation (growth fraction), expressed in all cell cycle phases except G0. - Related markers: PCNA, mitotic count, and phosphohistone H3 (a mitosis-specific marker). - A high Ki-67 index means high proliferation, higher grade, and a worse prognosis, but often better chemosensitivity. - Burkitt lymphoma has a Ki-67 of nearly 100%.