Given the high levels of co-morbidity across drugs of abuse and the plausible shared heritability underlying susceptibility to developing addiction in general, three GWAS have combined cases addicted to any illicit drug [18, 118, 119]. Two of these GWAS reported genome-wide significant and replicable associations. First, a GWAS of general substance dependence liability, based on factor analysis of DSM-IV symptoms, identified the intergenic SNP rs2952621 (P=1.8×10−8, N=2,322 European Americans) and replicated the finding in an independent dataset (P=0.02, N=2,647 European Americans) [118]. Rs2952561 is a SNP of unknown biological consequence that is located upstream of the uncharacterized gene LOC151121.
