While IIM typically does not feature dramatic longitudinal fluctuations of plasma C3 and C4 protein levels with disease activity as is the case in SLE,54 intragroup analyses revealed that patients with IIM with anti-Jo-1, MAA in general, anti-PM/Scl or anti-Ro had significantly lower mean complement protein levels than those without these autoantibodies. Here, PRB1 is linked to systemic lupus erythematosus.