ERAP1 and ankylosing spondylitis: Further evidence in this direction comes from genome wide association studies (GWAS) pointing out that both Endoplasmic Reticulum aminopeptidase 1 (ERAP1), which is in an epistatic relationship with the HLA-B27, and ERAP2, are risk factors for Ankylosing Spondylitis (AS) [15,16,17].