reported between the short allele of 5HTTLPR and CD with aggressive symptoms among adolescents ascertained through a substance abuse treatment program.20 However, another study failed to detect an association between this polymorphism and externalizing behavior in a sample of high-risk adoptees, although secondary analyses suggested that the short allele increased risk for externalizing behaviors in conjunction with a genetic diathesis for alcohol dependence.21 There has also been a negative report of association between the DAT1 gene and CD in adolescents.22 A number of other studies have examined CD symptomatology in the context of ADHD, also with mixed results. One recent study reported an association between the catechol-O-methyltransferase (COMT) valine/methionine polymorphism and CD symptoms in ADHD cases,23 whereas another study failed to replicate the association with CD symptoms in an independent sample of ADHD cases, although some evidence for association was detected with a subset of the aggressive CD symptoms.24 In addition, CD traits among ADHD probands have been evaluated in the GAIN-ADHD sample, with no markers meeting the criteria for genome-wide significance across any of the three CD problem traits that were examined.25
