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General laboratory information only; it is not medical advice and does not interpret any individual patient’s result.
Short answer
Ki-67 (often written Ki67) is a nuclear protein used as a proliferation marker. It is present in cells in every phase of the cell cycle except G0, so antibody staining labels cycling cells and leaves resting cells unstained. In breast cancer it is measured by immunohistochemistry (IHC) on tissue, and the result is a percentage of positive tumour cells. That percentage is only as comparable between laboratories as the preanalytics, antibody and scoring method behind it.
What does the Ki-67 label?
Ki-67 is a nuclear marker expressed in all phases of the cell cycle other than G0. A laboratory test page from NeoGenomics describes it as expressed from late G1 through S, M and G2 and absent in quiescent G0 cells, and links higher proliferative activity with more aggressive tumours and shorter disease-free survival. The stain therefore reports the fraction of cells that are cycling, not the speed of division and not the presence of cancer as such: normal proliferating tissue stains too.
Bench groups use the same marker in other formats, for example to count cycling cells in culture; see what a flow cytometer measures and cell counting and viability for the neighbouring techniques. As with any antibody readout, a stain is interpreted against controls, a point covered for blots in loading-control normalization.
How is Ki-67 assessed in breast cancer?
The International Ki67 in Breast Cancer Working Group (IKWG), formed in 2009 to address inter-laboratory inconsistency in Ki67 assessment, published recommendations in 2021 (Nielsen et al., JNCI 113(7):808-819, doi 10.1093/jnci/djaa201). In outline:
- Method: immunohistochemistry on formalin-fixed, paraffin-embedded tissue.
- What is counted: positive invasive carcinoma cells as a percentage of all invasive cancer cells. Staining intensity is not interpreted.
- Recommended scoring: the IKWG global method, which counts 100 nuclei per field type (using a scoring application) and combines them into a weighted global score.
- Hot-spot counting: showed higher variability and lower reproducibility than the global method.
Antibody clones and preanalytics
MIB1 is described as the most widely validated antibody. Others in use include 30-9, K2, MM1 and SP6; MM1 may perform poorly, possibly depending on the staining platform. The same paper notes that Ki67 is more sensitive to fixation and handling than ER or HER2 testing. The preanalytical points it lists form a practical checklist for laboratory staff:
| Variable | What the IKWG paper says |
|---|---|
| Prefixation delay | Minimise it; delays of 16 hours or more can lower values |
| Tissue slice thickness | 5-10 mm, as for ASCO/CAP HER2 and hormone-receptor testing |
| Fixative and time | Neutral buffered formalin, 6-72 hours; fixation of 3 hours or 14 days can lower values |
| Specimen type | Core biopsies preferred |
| Block age | Perform IHC within 5 years of paraffin embedding |
| Excluded tissue | No ethanol-fixed or decalcified tissue |
| Scoring | Calibrated, standardised visual scoring; hot-spot counting less reproducible |
The authors conclude that analytical validity is achievable with careful preanalytics and calibrated, standardised visual scoring, and they recommend participation in quality assurance and quality control programmes.
What is a low Ki-67 score? Why there is no universal cutoff
There is no single, universal cutoff. The IKWG states that Ki67 analysis should only be used to drive patient care for cases with 5% or less or 30% or more, and that agreement between laboratories in the 5-30% range is not acceptable. Its scope is prognosis in early-stage ER-positive, HER2-negative disease, to help decide whether adjuvant chemotherapy can be omitted. This is why searches for a “normal range” or for specific values such as 10, 20 or 50 do not have one answer: a number means something only with the method, the laboratory and the clinical context. Anyone with a personal result should discuss it with the treating clinician.
What is the most significant prognostic indicator for breast cancer?
The sources read for this page do not name a single most significant indicator. They do say that Ki67 is considered prognostic only in a narrow setting (early-stage ER-positive, HER2-negative disease) and has limited clinical utility beyond the extreme values above. Prognosis in practice combines several tumour and patient factors assessed by clinicians.
Where it matters for laboratories
- Preanalytic control (fixation time, delay, block age) matters more for Ki67 than for ER or HER2.
- Clone and platform should be validated and documented; MIB1 has the most validation behind it.
- Scoring should be calibrated and, ideally, checked through external quality assurance.
- NeoGenomics notes that computer-assisted image analysis has been validated only for breast cancer.
Common misunderstandings
- “Ki-67 shows how malignant a tumour is.” It labels cycling cells; it is one prognostic input in a narrow setting.
- “Intermediate values are interpretable.” The IKWG finds 5-30% concordance unacceptable.
- “Any hot spot will do.” Hot-spot counting was less reproducible than the global method.
- “The debate is settled.” A published reply (Zhang and Yang, PMC8562964) argues that further standardisation effort is wasteful and calls for alternative quantitative methods.
FAQ
What does Ki-67 stain?
The nucleus of cells that are in any cell-cycle phase except G0.
How is it assessed in breast cancer?
By IHC on fixed, embedded tissue, scoring the percentage of positive invasive tumour cells, preferably by the IKWG global method.
What is a low Ki-67 score?
No universal definition exists. The IKWG treats 5% or less as the reliable low end and 30% or more as the reliable high end.
Sources (read 2026-11-07)
- Nielsen TO et al. Assessment of Ki67 in breast cancer: updated recommendations from the International Ki67 in Breast Cancer Working Group. JNCI 2021;113(7):808-819. PMC8487652
- International Ki67 in Breast Cancer Working Group. ki67inbreastcancerwg.org
- NeoGenomics, Ki-67 test page
- Zhang and Yang, reply letter, PMC8562964
