GABA system genes were selected based on their inclusion in any of three sources: a panel constructed to include candidate genes for addiction-related phenotypes (Hodgkinson et al., 2008), an expert-curated list of addiction-pertinent genes (Saccone et al., 2009), and a database devoted to organizing genes by biological system pathways (Kanehisa, 1996). Genes were selected if any of these sources listed them as being involved in GABA-related activity; in this way 36 genes were selected. We examined markers within 5kb upstream (5’ direction) and 1kb downstream (3’ direction) of each gene, using NCBI build 36.1 annotation. For two small genes, GABRD (chr. 1) and GABARAP (chr. 17), no markers within this region were available, so these genes were excluded from subsequent analyses. Because GABAA subunit genes in the chr. 15q cluster, GABRA5, GABRB3, and GABRG3 lie on an imprinted chromosomal region, in which only paternally-transmitted copies of the genes are expressed (Meguro et al., 1997), markers on these genes would be inappropriate to assess using methods that do not account for the identity of the parent from whom each allele was
