Correlation of ID4 promoter methylation with ID4 expression on both mRNA and protein level. Coherence of ID4 promoter methylation and ID4 expression in three representative samples of primary breast cancer and corresponding normal tissue (matched pairs a, b, c). Normal tissues are labelled with N. Their corresponding tumours are labelled with T. For each sample, ID4 promoter methylation, ID4 mRNA expression and ID4 immunohistochemical staining are shown (original magnification: 400×). The Fold change (FC) of ID4 mRNA downregulation is calculated by the expression ratio of normal (N) and tumourous tissue (T) for each matched pair (a, b, c). Matched pair a: Unmethylated tumour showed very similar ID4 mRNA and protein expression compared to the corresponding normal breast tissue. Matched pairs b and c: Methylated tumours exhibited explicit loss of ID4 mRNA and ID4 protein in comparison to the corresponding normal breast tissues (N). See text for details.