COMT and nicotine dependence: A G > A substitution rs740603 in the intron 1 of COMT gene, predicted to alter transcriptional regulation (F-SNP), though failed to show any association with ADHD in Caucasian subjects from Finland [26] and Ireland [20], a haplotype consisting of the “A” allele was reported to provide protection towards nicotine dependence in the African-American population (P = 0.0005) [61].