HLA allele and haplotype distribution among Bulgarian patients with PV is similar to the already established predisposing associations with DRB1*14, DRB1*04:02, B*38, B*55, whereas DRB1*03:01 alleles and the corresponding haplotypes were significantly decreased in PV patients from the Bulgarian population. The gene discussed is HLA-DRB1; the disease is acquired polycythemia vera.