The Ki-67 protein is a cellular marker for proliferation. It is strictly associated with cell proliferation. During interphase, the Ki-67 antigen can be exclusively detected within the cell nucleus, whereas in mitosis most of the protein is relocated to the surface of the chromosomes.

Ki-67 protein is present during all active phases of the cell cycle (G₁, S, G₂, and mitosis), but is absent from resting cells (G₀).Ki-67 is an excellent marker to determine the growth fraction of a given cell population.

The fraction of Ki-67-positive tumor cells (the Ki-67 labelling index) is often correlated with the clinical course of cancer. The best-studied examples in this context are carcinomas of the prostate and the breast.