The relative roles of HLA class II variants including DRB1*03, DQA1*05, DQB1*02 and C4A deficiency on genetic predisposition to autoimmune diseases such as IIM are an unsolved enigma.17 47 59 Multivariate logistic regression analyses revealed that C4A deficiency was an independent risk factor for DM and JDM and that HLA-DRB1*03 was a prominent risk factor for IBM, while C4A deficiency and HLA-DRB1*03 contributed independently and interactively to an increased risk of PM. Here, C4A is linked to acquired idiopathic inflammatory myopathy.