CLOCK and cancer: To date, over 30 DNAm-based estimators of ‘epigenetic age’ have been constructed, including the “first generation” blood-based (Hannum, 2013) [24] and pan-tissue-based clocks (Horvath, 2013) [25] used to predict chronological age; the “second generation” of tools captured age-related and functional-phenotype modifications [26,27,28,29,30], and later development yielded cancer-specific epigenetic clocks that approximate a mitotic clock in normal and cancer tissues [31,32,33].