Ki-67 is a protein found inside cells that are actively growing and dividing. When it appears on a pathology report, it’s expressed as a percentage, telling you how many cells in a tissue sample were caught in the act of multiplying. A Ki-67 score of 10% means that out of every 100 cells examined, 10 were dividing. Doctors use this number most often in cancer care to gauge how aggressively a tumor is growing.
What the Ki-67 Protein Actually Does
Ki-67 is produced by cells during the active phases of the cell cycle, specifically during the later stages of growth and DNA copying, peaking right before and during cell division. Cells that are resting and not preparing to divide produce no detectable Ki-67 at all. This on/off behavior is what makes it so useful as a marker: if a cell is staining positive for Ki-67, it is either preparing to divide or already dividing.
The protein itself has a surprisingly physical job. When a cell divides, the membrane around its nucleus breaks apart, leaving chromosomes floating loose in the cell’s interior. Ki-67 coats the surface of those chromosomes and acts like a repellent, carrying a strong electrical charge that keeps individual chromosomes from clumping together into one tangled mass. Think of it like a coating that makes each chromosome slippery to the others. Later in division, when the cell needs chromosomes to regroup, Ki-67 switches behavior and draws them back together so a new nuclear envelope can form around them.
How a Ki-67 Score Is Calculated
Ki-67 is measured through a lab technique called immunohistochemistry. A thin slice of tissue, typically 4 to 5 micrometers thick, is placed on a glass slide, treated with an antibody that binds specifically to the Ki-67 protein, and then stained so that any cell containing Ki-67 turns a visible color under a microscope. The tissue must be preserved in formalin for 6 to 72 hours beforehand. Fixation shorter than 6 hours can produce unreliable results.
A pathologist then counts between 500 and 1,000 cells and records how many stained positive. The result is reported as a simple percentage: positive cells divided by total cells counted. So if 150 out of 1,000 cells are positive, the Ki-67 index is 15%.
Ki-67 in Breast Cancer
Breast cancer is where Ki-67 testing comes up most frequently. The score helps oncologists classify tumors and make treatment decisions, particularly when distinguishing between less aggressive luminal A tumors and more aggressive luminal B subtypes. A higher Ki-67 score generally signals a faster-growing tumor with a more aggressive biology.
There is no single universally agreed-upon cutoff between “low” and “high,” which can be confusing when you’re reading your own report. Research has examined categories ranging from under 10% through above 40%. One large study analyzing disease-free survival found that a cutoff of 29% was a meaningful threshold: patients with scores above that level had roughly 4.5 times the risk of recurrence compared to those below it. Many oncologists in practice use a cutoff somewhere between 20% and 30%, though your care team may interpret the number in the context of your full tumor profile rather than relying on a single threshold.
Ki-67 also plays a role in predicting how well a tumor responds to chemotherapy before surgery. Tumors with high Ki-67 scores respond to pre-surgical chemotherapy at higher rates (about 31%) compared to tumors with low scores (about 19%). That might sound counterintuitive: faster-growing cancers are in some ways more vulnerable to treatment. Chemotherapy targets dividing cells, so a tumor full of actively dividing cells gives the drugs more targets to hit. This does not mean a high score is “good news,” but it does mean the tumor is more likely to shrink with treatment.
Beyond Breast Cancer
Ki-67 is also a standard part of grading neuroendocrine tumors, including those found in the pancreas, lungs, and gastrointestinal tract. The World Health Organization grading system for neuroendocrine tumors relies heavily on Ki-67 to assign a grade from 1 to 3. Grade 1 tumors typically have a Ki-67 below 3%, grade 2 falls between 3% and 20%, and grade 3 exceeds 20%. In these cancers, the Ki-67 score directly shapes decisions about surgery, monitoring schedules, and whether systemic treatment is needed.
The marker appears in pathology reports for lymphomas, brain tumors, soft tissue sarcomas, and other cancers as well, though the specific cutoffs and clinical weight vary by cancer type.
Why Scores Can Vary Between Tests
One important limitation of Ki-67 is that the same tumor can produce different scores depending on how and when the sample was taken. Core needle biopsies, which are small tissue samples taken with a needle, consistently produce higher Ki-67 scores than surgical specimens removed later. This happens for a practical reason: larger surgical specimens take longer to reach the fixation solution in the lab. During that delay, cells continue dividing in an oxygen-starved environment, and the Ki-67 protein can degrade before it’s preserved. The result is a lower, potentially misleading reading.
Pathologist-to-pathologist variability is another known challenge. Two pathologists examining the same slide can arrive at somewhat different percentages, particularly in the intermediate range. An international working group formed in 2009 specifically to address this inconsistency. Their updated 2021 guidelines recommend standardized scoring methods and protocols for both manual and automated counting, but differences between labs have not been fully eliminated. This is one reason oncologists rarely make treatment decisions based on Ki-67 alone. It is typically interpreted alongside tumor size, hormone receptor status, genetic profiling tests, and lymph node involvement.
What Your Score Means for You
If Ki-67 appeared on your pathology report, the number reflects the growth speed of your tumor at the moment the sample was taken. A low score generally indicates a slower-growing cancer. A high score indicates faster growth, which often means more aggressive treatment is recommended but also that the cancer may be more sensitive to chemotherapy.
The score is one piece of a larger puzzle. In breast cancer, it often appears alongside estrogen receptor, progesterone receptor, and HER2 status. Together, these markers create a biological profile that guides your oncologist’s recommendations. Genomic tests like Oncotype DX or MammaPrint, which analyze the activity of multiple genes at once, have partly overlapped with Ki-67’s role, but Ki-67 remains widely used because it’s inexpensive, fast, and available in virtually every pathology lab in the world.