According to the existing data regarding CLOCK-related modulation of breast cancer development, we shall focus on transcriptional and epigenetic regulation of the core clock genes assuming that: 1) circadian genes can modulate cancer suppression via caretaker, gatekeeper and landscaper genes and direct modification of chromatin structure; 2) expression of the clock genes seems to be epigenetically modulated, 3) there is cross-talk between the clock genes and ER status. This evidence concerns the gene CLOCK and cancer.