Ki-67 antigen is a nuclear protein related to cell proliferation, which may be the key factor of cell proliferation. In addition, Ki-67 is related to ribosomal RNA transcription. Ki-67 is present in G1, S, M and G2 phases of cell division cycle, but not in G0 phase of cell quiescence. Ki-67 is used to judge the different types of tumors, because it exists in all proliferating cells (normal cells and tumor cells), and it is a very good marker to determine the proliferation rate of a given cell population.