AATK and Patent ductus arteriosus: To further validate the methylation level of the AATK gene in PDA patients from a validation cohort, quantitative DNA methylation analysis spanning seven CpG sites was performed to focus on this differentially methylated region in which these probes were located in the intron 1 of AATK. The methylation level of these CpG sites in tumor tissues was significantly increased compared to that in adjacent normal tissues (Fig. 2b).