HLA-DRB1 and acquired polycythemia vera: In both allele and phenotype analyses, DRB1*14 showed a remarkably high risk for PV occurrence (Pc < 0·00001, OR 6·47, 95% CI 4·52–9·26 and Pc < 0·00001, OR 9·68, 95% CI 4·47–20·98, respectively).