Originally discovered employing mouse monoclonal antibody against a nuclear antigen from Hodgkin's lymphoma-derived cell line, this non-histone protein was named Ki67 after researcher's location (Gerdes and colleagues), Ki for Kiel University in Germany and 67 referring to the clone number on the 96-well plate. It interacts with KIF15 as well as MKI67IP, and is suggested to be involved in cell cycle regulation. Ki67 is a large protein with expected molecular weight of about 395 kD and has a very complex localization pattern within the nucleus, one which changes during cell cycle progression. Its expression occurs specially during late G1, S, G2 and M phases of the cell cycle, while in cells undergoing G0 phase, Ki67 remains undetectable. Ki67 undergoes phosphorylation/dephosphorylation during mitosis, is susceptible to proteases and its structure implies that its expression is regulated by proteolytic pathways. Ki67 is associated with nucleolar DFC (dense fibrillary component) and its regulation appears to be tightly controlled (estimated half life is 60-90 min, regardless of the cell position in the cell cycle), presumably by precise synthesis and degradation systems involving proteasome, a protease complex. Due to its association with cell divison process, Ki-67 is routinely used as cellular proliferation marker of solid tumors as well as certain hematological malignancies, and a correlation has been demonstrated between Ki-67 index and the histopathological grade of cancers.