Despite this moderate (50%), yet robust, estimate of heritability and evidence for genetic co-aggregation, gene identification for addiction has been extremely challenging. A majority of these gene-finding efforts have relied on candidates with a priori biological support. Broadly, these genes can be characterized as those participating in the experience of psychoactive effects of the substance (i.e., genes encoding proteins affecting neurotransmitter signaling) and those regulating their bio-availability and clearance (e.g., metabolizing genes), as shown in Fig. 1.
