PER2 and cancer: In order to achieve a systematic understanding of circadian clock in cancer, we define a core subset of clock family genes including 7 positive regulators (CLOCK, NPAS2, ARNTL, ARNTL2, RORA, RORB, and RORC) and 7 negative regulators (PER1, PER2, PER3, CRY1, CRY2, NR1D1, and NR1D2) (Table S1) (13).