A fine-mapping analysis of PV risk in the major histocompatibility complex region showed three independent variants predisposed to PV using stepwise analysis: HLA-DRB1*14:04 (P = 2.47 × 10<sup>-38</sup>, odds ratio = 6.28), rs7454108 at the TAP2 gene (P = 2.78 × 10<sup>-12</sup>, odds ratio = 3.25), and rs1051336 at the HLA-DRA gene (P = 3.06 × 10<sup>-6</sup>, odds ratio = 0.33).