In a study of 95 white patients with JDM, we showed that C4A deficiency was a strong risk factor for JDM.33 How complement C4 genetic diversity contributes to disease predisposition in different forms of IIM, the development of MSA and/or MAA and the relative roles of HLA-DRB1*03 and C4A deficiency in IIM have yet to be assessed, however. This evidence concerns the gene C4A and acquired idiopathic inflammatory myopathy.