Circadian locomotor output cycles kaput (CLOCK) is a transcription factor that activates transcription of clock-controlled genes by heterodimerizing with BMAL1 and binding to E-box elements on DNA. The expression of CLOCK in HCC tissues was much higher than that in normal liver tissues, and the high expression of CLOCK indicated the poor prognosis. The knockdown of CLOCK in HCC cells could inhibit the proliferation of HepG2 cells. The expression of circadian gene CLOCK in ovarian cancer cells was strongly associated with cisplatin resistance. The upregulation of circadian gene CLOCK in ovarian cancer cells may reduce its sensitivity to cisplatin treatment.