[5][6][7] Antigen KI-67 is a nuclear protein that is associated with cellular proliferation and ribosomal RNA transcription.

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.

The fraction of Ki-67-positive tumor cells (the Ki-67 labeling index) is often correlated with the clinical course of cancer.

The best-studied examples in this context are prostate, brain and breast carcinomas, as well as nephroblastoma and neuroendocrine tumors.

The Ki-67 protein was originally defined by the prototype monoclonal antibody Ki-67,[17] which was generated by immunizing mice with nuclei of the Hodgkin lymphoma cell line L428.