HLA-DRB1 and acquired polycythemia vera: DRB1*08 frequencies were significantly higher in patients with PV than in controls in both the allele (Pc = 0·03, OR 2·25, 95% CI 1·07–4·70) and phenotype (Pc = 0·0003, OR 2·46, 95% CI 1·51–4·01) analyses, but heterogeneity (I2 = 54%) was observed in the allele group.