We observed a significant reduction of NDRG4 expression levels by up to 43% in the Q3 and Q4 quartiles when compared with the Q1 (non-methylated) group (p < 0.01) (Fig. 2b).17 In addition, breast cancer clinical stage or histological subgroups (TN, HER2+ or ER+/PR+) did not significantly differ with regard to NDRG4 promoter methylation levels (Supplementary Fig. S2). Here, ESR1 is linked to breast cancer.