Evidence from initial research implicated PER1 as a regulator of the circadian clock(Han et al., 2016; Korge et al., 2018; Repouskou et al., 2016; Reinhardt et al., 2012; Riley et al., 2008); however, recent evidence has expanded the function of PER1 to include the promotion of apoptosis in cancer cells (Han et al., 2016; Korge et al., 2007). This evidence concerns the gene CLOCK and cancer.