The presence of the minor allele in SNP rs7131465, located in the 5′-terminal untranslated region (5′ UTR) of the DRD2 gene, seems to be a strong risk factor of developing ASD/ADHD overlap vs. a singular clinical disease, that is ASD or ADHD (OR = 2.7; 95%CI = 1.4701–5.024; corrected P-value = 0.003 and OR = 2.8; 95%CI = 1.5581–5.3447; corrected P-value = 0.003, respectively). This evidence concerns the gene DRD2 and attention deficit-hyperactivity disorder.