CLOCK and cancer: We studied four epigenetic signatures reflecting different aspects of carcinogenesis and cancer prevention: (i) a signature reflecting carcinogen exposure; (ii) a signature indicating cell-type composition18; (iii) a mitotic clock based on methylation of polycomb group target (PCGT) genes18; and (iv) a signature indicating methylation at progesterone receptor binding sites (PRBS)5 (Fig. 1a).