Variants within and distal to the major histocompatibility complex (MHC) locus were associated with BD at genome-wide significance. The most highly associated SNP was rs13195402, 3.2 Mb distal to any HLA gene or the complement component 4 (C4) genes (Supplementary Fig. 4). Imputation of C4 alleles using SNP data uncovered no association between the five most common structural forms of the C4A/C4B locus (BS, AL, AL-BS, AL-BL, and AL-AL) and BD, either before or after conditioning on rs13195402 (Supplementary Fig. 5). While genetically predicted C4A expression initially showed a weak association with BD, this association was non-significant after controlling for rs13195402 (Supplementary Fig. 6).
