Ki-67 in Immunohistochemistry: What the Marker Shows

What Ki-67 staining shows

Ki-67 is a nuclear protein associated with cells in active phases of the cell cycle and is generally absent from quiescent G0 cells. In IHC, staining is used as a measure of the proliferating fraction of the assessed cell population. The stain describes cycling-cell labeling rather than establishing a diagnosis.^[1][2]

What a Ki-67 labeling index means

A Ki-67 labeling index is the percentage of selected tumor-cell nuclei scored positive among the tumor cells counted. The number is meaningful only alongside the tissue and tumor area sampled, staining quality, and the scoring approach. Counting a high-staining area can give a different result from a broader assessment.^[1][2]

Why methods matter

The International Ki67 Working Group emphasizes preanalytic handling, standardized scoring, and quality assurance because results can vary between observers and laboratories. Its breast-cancer guidance is specific to that setting; a single percentage should not be applied as a universal cutoff across tumor types.^[1]

When comparing two Ki-67 reports, the stain and counting method matter alongside the number. The working group specifically evaluated variation caused by field selection and by choosing a high-staining 'hot spot' instead of a broader count.^[1]

Relevant ReagentScope records

The ReagentScope Ki-67 marker page lists two verified Bio SB clone-level records: the Ki-67 EP5 product page and Ki-67 RM360 product page. Those manufacturer-listed specifications do not define how a laboratory should score a specimen.^[3][4]