PCNA and neoplasm: Ki-67 is a cell proliferation marker that is expressed in all phases of the cell cycle, except the G0 phase.22, 23 PCNA, a 36-kD DNA polymerase delta auxiliary protein involved in the proliferation of neoplastic and non-neoplastic cells,24, 25 is specifically expressed in proliferating cell nuclei and is commonly used to measure tumor cell proliferation.