CLOCK and hepatocellular carcinoma: Wee1 encoding an essential kinase in the G2/M checkpoint and p21Cdkn1a/Waf1/Cip1 (referred to as p21 hereafter) encoding a cyclin-dependent kinase inhibitor that negatively regulate cell cycle progression are transcriptional targets of the circadian clock circuit that contribute to Bmal1/Clock-promoted HCC cell proliferation.