HLA-B27 is strongly associated with ankylosing spondylitis (Chen et al. 2014), and genetic studies indicated that certain ERAP1 allotypes are more associated with ankylosing spondylitis than others, due to their contribution to peptide trimming (Reeves et al. 2014a, Reeves et al. 2014b). Here, ERAP1 is linked to ankylosing spondylitis.