The present study can be summarized as follows: (i) the frequencies of DRB1*01:01, DRB1*04:10, and DRB1*15:02 were significantly higher in the Japanese patients with IBM than in healthy controls (Corrected P = 0.00078, 0.00038 and 0.0046); (ii) there was a weak association between the DRB1*01:01 allele and clinical features (severe leg muscle weakness and muscle atrophy); and (iii) HCV infection and anti-cN1A antibodies were not associated with HLA-DRB1 alleles. This evidence concerns the gene NT5C1A and inclusion body myositis.