Two years after the long-term follow-up (4.5 years after the pretest), we attempted to contact the study families to obtain youths’ and parents’ DNA from saliva samples. Of the original sample assessed at posttest, 88% (n = 566) of the families were located and one or more members of the family agreed to provide a DNA sample. This resulted in successful genotyping at DRD4 for 448 youths and 436 primary caregivers. Among youths, 171 carried at least one DRD4 7-repeat allele and 169 carried two 4-repeat alleles. Among caregivers, 175 carried at least one DRD4 7-repeat allele and 169 carried two 4-repeat alleles. Participants with other infrequent alleles (e.g. 2,2; 2,3; 2,4) were excluded (n = 108); most of these individuals carried the 4-repeat allele in combination with a rare variant. Three additional participants were excluded because they could not be adequately characterized with regard to 5HTT.
