Imputation has two primary uses: fine-mapping existing association signals and detecting novel associations. GWAS have had only a few examples of successful fine-mapping to single causal variants27,28, often because of extensive haplotype structure within regions of association29,30. We find that, in Europeans, each previously reported GWAS signal31 is, on average, in linkage disequilibrium (r2 ≥ 0.5) with 56 variants: 51.5 SNPs and 4.5 indels. In 19% of cases at least one of these variants changes the coding sequence of a nearby gene (compared to 12% in control variants matched for frequency, distance to nearest gene and ascertainment in GWAS arrays) and in 65% of cases at least one of these is at a site with GERP>2 (68% in matched controls). The size of the associated region is typically <200 kb in length (Figure 5b). Our observations suggest that trans-ethnic fine-mapping experiments are likely to be especially valuable: among the 56 variants that are in strong linkage disequilibrium with a typical GWAS signal, ~15 show strong disequilibrium across our four continental groupings (Table S15). Compared to earlier catalogs, our current resource
