We leveraged a robust collection of biospecimens and clinical data for patients with IIM recruited by Investigators of the Myositis Genetics Consortium (MYOGEN) from the UK, Sweden, the Czech Republic, Belgium and the USA, plus geographically matched healthy controls to investigate the GCN variations of total C4 (C4T), C4A, C4B, long C4 genes (C4L) and short C4 genes (C4S) in disease susceptibility for IIM and its four major subtypes. This evidence concerns the gene C4A and acquired idiopathic inflammatory myopathy.