Stein et al. (2005), children with the 9/10 and 10/10 genotypes showed significant improvement in ADHD symptoms following treatment as measured by the CGI-Parents scale when compared to 9/9 children; no difference was observed between the 9/10 and 10/10 groups. The 10-repeat allele was also associated with better treatment response to methylphenidate following longer term treatment in children diagnosed with ADHD using family-based association testing (Bellgrove et al. 2005). In addition to these studies, three studies have observed no association between the 3′-UTR VNTR polymorphism and methylphenidate response (da Silva et al. 2010; McGough et al. 2006; Roman et al. 2001). Therefore, the reported effects of the 3′-UTR VNTR on treatment response are contradictory and warrant further investigation.
