PER2 and cancer: Moreover, we used Pearson correlation to analyze the correlation between the expression and methylation level of rhythm genes in 13 cancer types, and revealed that the methylation level of PER1, PER2, PER3, NPAS2, BHLHE40, C1orf51, and ARNTL2 are significantly negatively correlated with their expression level, respectively (Figure 3B), which may partially explain the expression changes of these rhythm genes in cancer samples.