ISG15 and polymyositis: Consistent with a previous study that examined possible related signaling pathways shared in the pathogenesis of several systemic autoimmune diseases (SAID) such as dermatomyositis, polymyositis, rheumatoid arthritis, and SLE, a subset of five viral-related differentially expressed genes (i.e., RSAD2, IFIT3, ISG15, STAT1, and EIF2AK) was detected in peripheral blood of SAID probands and their unaffected twins (50).