Although GWAS have generally failed to identify many of the genes that were previously identified by candidate gene studies, this is not to say that those findings are necessarily erroneous. It would appear that they may be involved in particular endophenotypes and particular populations. In some cases, these effects may be quite strong. Thus, genetic variation in the Oprm1 gene is generally associated with sensitivity to opiates, including both analgesic and addictive properties of this class of drugs, as well as others (for review see (Ikeda, et al., 2005)). The genetic differences underlying these associations are likely to be quite heterogeneous as more than 100 polymorphisms in the Oprm1 gene have been demonstrated, many of which produce functional consequences on OPRM1 expression, binding affinity or other aspects of OPRM1 function.
