PCNA and lung cancer: 16HBE, a kind of bronchial epithelial cells, had the slowest proliferation rate compared with LLC and A549 (mice and human lung cancer cells) as shown in Fig. 1A. Moreover, the expression of Ki67 and PCNA (proliferation-associated proteins) was lower than LLC and A549, suggesting that lung cancer cells proliferated rapidly (Fig. 1B–D).