Antigen Ki-67 is a nuclear protein that is associated with and may be necessary for cellular proliferation. Furthermore, it is associated with ribosomal RNA transcription. Ki-67 was present in the nuclei of cells in the G1, S, and G2 phases of the cell division cycle as well as in mitosis. Quiescent or resting cells in the G0 phase did not express the Ki-67 antigen. Ki-67 protein was used as diagnostic tools in different types of neoplasms because it was present in all proliferating cells (normal and tumor cells) and was an excellent operational marker to determine the growth fraction of a given cell population.