We used PRS-CSx, a method that couples genetic effects and LD across ancestries via a shared continuous shrinkage (CS) prior, to calculate the posterior effect sizes for SNPs mapped to HapMap3. Three sets of AUD GWAS summary data were use as input and corresponding posterior effect sizes in each ancestry were generated: EUR (without AUDIT–P from UKB, Neffective = 352,373), AFR (Neffective = 105,433) and LA (Neffective = 30,023). Three sets of AUD PRS based on the posterior effect sizes were calculated for UKB–EUR1 and UKB–EUR2 individuals using PLINK, following standardization (zero mean and unit variance) for each PRS. For each related pair (≥3rd degree, kinship coefficient ≥0.0442 as calculated by UKB), we removed the individual with the lower AUDIT–P score, or randomly if they had the same score, leaving 123,565 individuals in UKB–EUR1 and 17,401 in UKB–EUR2. Then, we ran linear regression for AUDIT–P in UKB–EUR2 as a validation dataset using PRSEUR, PRSAFR and PRSLA as independent variables. The corresponding regression coefficients were used as weights in the test dataset (UKB–EUR1) to calculate the final PRS: PRSfinal = ωEUR
