PLK1 and lung cancer: Therefore, we first determined whether UBE2C knockdown would affect the levels of these substrates, including PLK1, cyclin B1, and securin, and found a minor, if any, effect in 4 lung cancer cell lines harboring mutant Kras (Supplemental Figure 3A), suggesting that these substrates are not actively involved in growth suppression induced by UBE2C knockdown.