Further cell cycle analysis of cancer cells showed that zebularine inhibited tumor cell growth, mainly by suppressing cell cycle G1 phase and expression of cell proliferation-related genes Pcna; however, we did not see the consistent change in the mRNA expression of Mcm6 and Mki67 by zebularine (Fig. 4c, d and Supplementary Fig. 5a). The gene discussed is PCNA; the disease is cancer.