This study demonstrates that the down-regulation of PER1 in cancer cells can result in the robust up-regulation of PER3, TIM, RORα and REV-ERBα transcripts, while the expression of PER2, DEC1, DEC2, CRY1, CRY2 and NPAS2 is prominently down-regulated in vitro and in vivo. This evidence concerns the gene NPAS2 and cancer.