Exclusion of entities potentially misdiagnosed as ASS by appropriate criteria (e.g., sarcoplasmic MxA expression for DM and clinicopathological criteria for IBM) help increase diagnostic performance of HLA‐DR to 95.4% specificity, 61.2% sensitivity, 85.9% PPV and 84.2% NPV. The gene discussed is MX1; the disease is inclusion body myositis.