the population at random. However, as OPRM1 lies on chromosome 6, OPRK1 on chromosome 8, and OPRD1 on chromosome 1, it is unlikely that oversampling for 118G carriers biased the sample for the OPRK1 and OPRD1 markers. Next, given that participants guessed their current medication (naltrexone or placebo) above chance levels, it is possible that medication expectancies may have influenced results. Finally, the tSNPs examined were selected to provide sufficient coverage of the genes of interest, and it remains unknown where the true genetic “signal” lies within these genes as well as their mode of inheritance.
