For sequencing purposes, genomic DNA was collected and isolated from buccal cells following published procedures (Freeman et al., 1997, Lench et al., 1988, Walker et al., 1999). An ABI PRISM 7500 instrument (Applied Biosystems, Inc. Foster City, CA) was used to conduct 5′-nuclease (TaqMan) assays of the opioid receptor SNPs using assays commercially available from Applied Biosystems.