SLC22A1 and hepatocellular carcinoma: When we correlated DNA methylation with SLC22A1 mRNA expression in the total group of 22 HCC samples (HCC study population, set 2; Tables S1 and S3 in Additional file 1), a significant inverse correlation between SLC22A1 mRNA expression and individual CpG sites within the SLC22A1 promoter region was found (Figure 3e; Table 1).