Ki-67

Every time a cell divides, it produces a protein called Ki-67 as part of the process. Cells at rest do not make it. Pathologists take advantage of this by staining tumor biopsy samples with a marker that lights up wherever Ki-67 is present, then calculating what percentage of the cancer cells are producing it. That percentage — the Ki-67 index — is essentially a direct measurement of how actively the tumor is proliferating at the time the sample was taken.

A low Ki-67 score (generally below 15 to 20 percent, though cut-offs can vary between laboratories) suggests the tumor is growing relatively slowly. A high score indicates aggressive, fast-dividing cancer. In the context of breast cancer, Ki-67 is one of several markers that inform the overall picture of tumor biology alongside grade, hormone receptor status, and HER2 status. It is also a key input in some genomic tests, like Oncotype DX and Mammaprint, which use patterns of gene activity — including proliferation markers — to predict the likelihood of recurrence and the benefit of chemotherapy.

Ki-67 is particularly useful when making treatment decisions for hormone receptor-positive, HER2-negative breast cancers: a low Ki-67 in this setting often supports a recommendation of hormone therapy alone, while a high Ki-67 may indicate that chemotherapy should be added. Your oncologist will interpret your Ki-67 result in the context of all your tumor's characteristics, not in isolation.

When your oncologist mentions Ki-67, they are trying to understand the tempo of your cancer — and that matters for treatment planning. A fast-moving tumor may respond differently to chemotherapy than a slow-growing one, and knowing that information can help avoid both over-treatment (with its associated side effects) and under-treatment (which could leave active cancer inadequately addressed).

It is worth asking what your Ki-67 score is and how it fits into the overall picture your care team is building from your pathology results. Taken together with hormone receptor status, HER2 status, grade, and stage, Ki-67 helps your oncologist personalize your treatment rather than apply a one-size-fits-all protocol. If a genomic assay like Oncotype DX is being considered for your case, Ki-67 is part of the underlying biology those tests are measuring — another reason to understand this marker as part of your overall tumor profile.