The Ki-67 antigen is a human nuclear protein, which is expressed in all active parts of the cell cycle (G1, S, G2 and mitosis), but absent in resting cells (G0).

In contrast to many other cell cycle-associated proteins, the Ki67 antigen is consistently absent in quiescent cells and is not detectable during DNA repair processes. Thus, the presence of Ki67 antigen is strictly associated with the cell cycle and confined to the nucleus, suggesting an important role of this structure in the maintenance and/or regulation of the cell division cycle.