Complement-mediated destruction leading to vasculopathy in dermatomyositis has been well-established,1 6 47 48 and we and others have demonstrated C4A genetic deficiency or low GCN of C4T in JDM.33 48 Demonstration of low C4T or C4L GCNs and C4A deficiency as genetic risk factors for DM, PM and IBM are novel findings of this work. The gene discussed is C4A; the disease is inclusion body myositis.