(A) Correlation between ID4 expression and ID4 promoter methylation in human breast cancer. Box plot analysis illustrating the loss of ID4 expression in relation to ID4 promoter methylation in primary human breast cancer. The Y axis indicates the factor of ID4 mRNA downregulation in breast cancer specimens (n = 46) relative to a normal breast standard (a pool of 12 normal breast tissue samples) as the fold change (FC) N/T. Unmethylated tumours exhibited ID4 expression (median FC = 1.3) very similar to normal breast cells. In contrast, methylated breast cancer specimens displayed an increased loss of ID4 expression (median FC = 12.3). Horizontal lines: group medians; boxes: 25–75% quartiles; vertical lines: range, peak and minimum. (B) Kaplan-Meier analysis of patients' recurrence-free survival (RFS) in relation to ID4 promoter methylation. Distribution of time (months) and tumour-related death among 115 breast cancer patients with positive (lower graph) or negative (upper graph) ID4 promoter methylation state is shown. Patients harbouring an ID4-methylated tumour have an estimated mean RFS time of 80 months (95% confidence interval: 67–93 months) compared with 101 months (95% confidence interval: 87–115 months) for patients without ID4 tumour methylation. See text for details.