PCNA and central nervous system cancer: PCNA was initially recognized as a proliferating cell-specific antigen expressed in cell nuclei during the S phase of the cell cycle; later studies and a systematic review show that increased PCNA expression is directly associated with malignancy, poor survival, and advanced GBM stage, and the marker has been proposed as a useful diagnostic or prognostic biomarker or an effective therapeutic target in glioma [61].