HIF1A and cancer: Further, we utilized the Spearman correlation analysis to compute the correlation between the gene expression and methylation level of HIF genes across cancer types and observed that the methylation level of HIF1A, ARNT, EPAS1, ARNT2, HIF3A, and ARNTL are significantly negatively correlated with their expression level, respectively (Figure 4(b)), which may partially explain the abnormal expression of these HIF genes in cancer samples.