The expression of PER1, PER2, and CRY2 decrease in human fibrosarcoma and in undifferentiated pleomorphic sarcoma (Rivera-Reyes et al., 2018).The expression of BMAL1 and CLOCK are reduced in ovarian cancer due to the methylation of CpG sites on gene promoter regions (Gutiérrez-Monreal et al., 2016). This evidence concerns the gene PER2 and ovarian cancer.