Genetic factors may have an impact on the development of SUDs. Presently, however, the exact mechanisms through which genes affect substance use are not clear, but evidence suggests a combination of several genes may be involved. Numerous possible candidate genes have been identified (see Chapter 2, "Genetics of Addiction," this volume). In this section, we present research pertinent to the interplay between genes and the environment, based on the work of Sir Michael Rutter and his colleagues (2006). By way of illustration, we present three types of gene–environment interplays. The first type of interplay refers to gene–environment interactions, which are based on the premise that drug addiction is the end result of a series of processes that interact to produce a particular outcome, above and beyond the separate genetic and/or environmental factors involved. Caspi et al. (2002), for instance, demonstrated that maltreated children with a genotype encoding high levels of monoamine oxidase A expression were less likely to develop antisocial behaviors in adulthood than maltreated children who did not have this genotype.