MPO/ANCA-positive EGPA has a significant association with HLA class II DQ haplotype, which is shared with the other MPO-AAV (i.e., microscopic polyangiitis, MPA), while ANCA-negativity is associated with GP33 and IL5/IRF1 loci, indicating a possible mucosal/barrier dysfunction origin (25). This evidence concerns the gene IRF1 and microscopic polyangiitis.