AbstractObjectiveTo compare the autoantigenicity of the recently described N‐terminally elongated PM–Scl‐75 protein with that of PM–Scl‐100 and the originally defined PM–Scl‐75 polypeptide, and to determine its value for analyzing sera from patients with the polymyositis (PM)/scleroderma overlap syndrome.MethodsSerum samples obtained from patients with the PM/scleroderma overlap syndrome and from patients with several other diseases were analyzed for the presence of autoantibodies reactive with recombinant PM–Scl‐100 and PM–Scl‐75 (both the original and the longer form) proteins, in an enzyme‐linked immunosorbent assay (ELISA).ResultsAutoantibodies recognizing the longer PM–Scl‐75 protein isoform were detected in 28% of the patients with PM/scleroderma. This percentage is slightly higher than that for PM–Scl‐100 (25%) and is significantly higher than that for the previously defined PM–Scl‐75 protein (11%). In addition, we identified a significant number of patients who had anti–PM–Scl‐75 but not anti–PM–Scl‐100 antibodies. This finding contrasts with what has been previously reported for the shorter version of the PM–Scl‐75 protein.ConclusionOur data indicate that use of the long PM–Scl‐75 isoform in addition to PM–Scl‐100 in ELISAs significantly increases the number of patients in whom anti–PM‐Scl autoantibodies can be detected.