To date, GWAS of alcohol (Table 2) as well as tobacco use (Table 3) using individual SNPs as well as PRS have reported modest evidence for the role of genetic influences on dependence-related measures. Two recent studies detected significant associations of small effect size (i.e., PRS accounting for less than 1% of the total variance) between polygenic risk scores and alcohol dependence (Kapoor et al., 2016; Salvatore et al., 2014). Further, an association was reported for age of onset for regular drinking that also accounted for less than 1% of the total variance (Vink et al., 2014). GWAS studies using PRS also emphasize the importance of the role of genetic influences in stage of SUD progression. However, results differ across different measures of SUD stages. Two recent studies noted significant associations with measures of dependence rather than initiation in both alcohol as well as tobacco use (Belsky et al., 2013; Kapoor, et al., 2016). Therefore, as identified in twin studies, genetic influences may be more important for maintaining dependence and its related neurobiology rather than early use
