These findings are relevant, as PM and IBM have been presumed to be disorders of cell-mediated immunity caused by target tissue cytotoxicity or destruction.1 The prevalence of low GCNs of C4T and C4L, C4A deficiency, and the presence of myositis-related autoantibodies in these diseases suggests that additional humoral immune effectors play a role in the pathophysiology of PM and IBM. The gene discussed is C4A; the disease is inclusion body myositis.