HLA-DRB1 and acquired polycythemia vera: In agreement with HLA-DRB1 allele distribution, two haplotypes, HLA-DRB1*04:02/DQB1*03 and DRB1*14/DQB1*05 were positively associated with PV and therefore, considered as predisposing, while the haplotypes DRB1*03:01 / DQB1*02 and DRB1*11/DQB1*03 showed significantly decreased frequency in the patients’ group (Table 5).