The exosome complex is the target of autoantibodies in patients suffering from various autoimmune diseases. These autoantibodies are mainly found in people that suffer from the PM/Scl overlap syndrome, an autoimmune disease in which patients have symptoms from both scleroderma and either polymyositis or dermatomyositis. Autoantibodies can be detected in the serum of patients by a variety of assays. In the past, the most commonly used methods were double immunodiffusion using calf thymus extracts, immunofluorescence on HEp-2 cells or immunoprecipitation from human cell extracts. In immunoprecipitation assays with sera from anti-exosome positive sera, a distinctive set of proteins is precipitated. Already years before the exosome complex was identified, this pattern was termed the "PM/Scl complex". Immunofluorescence using sera from these patients usually shows a typical staining of the nucleolus of cells, which sparked the suggestion that the antigen recognized by autoantibodies might be important in ribosome synthesis. More recently, recombinant exosome proteins have become available and these have been used to develop line immunoassays (LIAs) and enzyme linked immunosorbent assays (ELISAs) for detecting these antibodies.