Idiopathic inflammatory myopathies (IIMs) are rare systemic inflammatory diseases of partly unknown pathogenesis.1 Onset and progression of IIM is influenced both by environmental and genetic factors, but the exact interplay between these factors remains unclear.1 2 Recently, genome-wide association studies (GWASs) confirmed that alleles in the human leucocyte antigen (HLA) 8.1 ancestral haplotype—HLA-DRB1*03:01 and HLA-B*08:01—are important genetic risk factors for IIM.3–6 Other genes outside the HLA region such as PTPN22 have also been suggested. Here, PTPN22 is linked to acquired idiopathic inflammatory myopathy.