a polygenic risk score based on 4 of the top-SNPs from the 15q25.1 region and 2 SNPs from another region (19q13.2). This score was unrelated with smoking initiation, but the individuals with a high score were more likely to convert to heavy, persistent smoking (43). Another study incorporated a SNP score of 92 top-SNPs (based on meta-analyses (23)) in a developmental model of CPD. The SNP score was associated with CPD, but not to the frequency of alcohol use at different ages (44). Our results suggested that, besides the top-SNPs from the meta-analyses, a large number of SNPs with all small individual effect sizes contribute to substance (ab)use.
