We previously showed that changes in methylation of the 5’-UTR in the DAT1/SLC6A3 gene can be used as a biomarker for the prognosis of initial severe ADHD: treatment-resistant severe ADHD children were characterized by methylated CpG 1 in particular, while methylated CpGs 2 and 6 were then found in children who improved after the therapy. The gene discussed is SLC6A3; the disease is attention deficit-hyperactivity disorder.