Ki-67 is a nuclear, non-histone protein that is expressed only during active phases of the cell cycle (G1, S, G2 and M), but not in the resting phases (G0 and G1 early phase). Although the antigen has also been associated with ribosomal RNA transcription, it is strongly linked to cell proliferation and has thus been indicated as an effective marker in grading the proliferation rate of tumors, including those of the brain, breast, cervix, and prostate.