The Ki-67 antigen is a nuclear, non-histone protein found in proliferating cells. Due to its presence during cell division, Ki-67 is a great target for tagging tumors that are growing quickly, even though its exact function is mostly speculated and still needs to be identified. The immunohistochemical labeling of Ki-67 antigen, given its correlation with cell proliferation, may be helpful in determining the growth fraction of a lesion.