HLA-DRB1 and acquired polycythemia vera: In pemphigus vulgaris (PV), site-directed mutagenesis of either of the DRB1*0402 residues 70Asp and 71Glu to the corresponding DRB1*04:04 residues 70Gln and 71Arg ablated presentation of and immune response to the desmoglein antigen DG(190–204); DRB1*04:02 is risk for PV and DRB1*04:04 is protective[35].