PCNA and neoplasm: The approximate order of expression (or appearance) of these markers in freely proliferating cells is as follows: the p21 and p27 cyclin dependent kinase (CDK) inhibitors are specific to cell cycle arrest and exit (p27 is also regarded as a tumor quiescence marker)(12), Cyclin D1 (CCND1) to G1/S cells, Cyclin E1 (CCNE1) and the DNA licensing factors PCNA and Geminin to S phase cells, Cyclin A2 (CCNA2) and Cyclin B1 (CCNB1) to G2/M cells, and phospho-histone H3 (pH3) to mitotic cells (Fig. 3f; Supplementary Fig. 4c).