When testing the putative cis-eQTL SNPs for association with heroin addiction, many of the cases used in our study were derived from the UHS, where no study controls were available, and they were compared to a set of population controls obtained via dbGaP. This study design enabled us to create a large sample size for heroin addiction association testing. Stringent quality control was implemented to reduce the chances of introducing artifactual biases, and our top findings were corroborated in independent cohorts including DSM-IV-assessed cases and study controls. Greater power for identifying variants associated specifically with heroin addiction could be achieved with a large sample of controls who had used heroin but never became addicted. No such large sample of exposed but never addicted controls exists, but their absence does not diminish the observed associations of rs3778150 and other intron 1 SNPs with heroin addiction.
