The Reviron model proposes that shared epitope positive HLA-DRB1 alleles which have a very positive charge in their P4 pocket (HLA-DRB1*0401, HLA-DRB1*0404, HLA-DRB1*0405, HLA-DRB1*0408, HLA-DRB1*0101) predispose to RA whereas among shared epitope negative alleles, those which have a positive charge in their P4 pocket, called XP4p (HLA-DRB1*03, HLA-DRB1*09, HLA-DRB1*14, HLA-DRB1*15, HLA-DRB1*16), are neutral and those with a negative or neutral charge in their P4 pocket, called XP4n (HLA-DRB1*07, HLA-DRB1 *08, HLA-DRB1*11, HLA-DRB1*0402), protect against RA [4]. The gene discussed is HLA-DRB1; the disease is rheumatoid arthritis.