Autoantibodies against various cellular components are found in sera from patients with connective tissue diseases. These autoantibodies have been demonstrated to be associated with certain diseases and clinical manifestations, and give us useful information for clinical practice. The development of new technologies for detecting autoantibodies has facilitated identification of more than 100 of autoantibodies and their target autoantigens. Structure and function of target autoantigens have been

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... recently clarified; they are mostly intracellular enzymes and regulatory factors necessary for important biological function involved in gene replication, repair, transcription, RNAprocessing and protein translation. Thus, the studies of antoantibodies are useful not only in clinical medicine but also in basic cellular and molecular biology. About two decades ago, the author encountered an interesting case that initially revealed inflammatory myositis and later developed Raynaud's phenomenon and skin thickening on her upper extremities. It was noted that this patient's serum contained newspecific autoantibodies on Ouchterlony's double immunodiffusionassay, and we termed the newautoantibody system as anti-"Ku" from the initial of the prototype patient name (1). In screening of a number of various patient sera, anti-Ku antibodies were detected mostly in overlap syndrome, especially overlapping with scleroderma (SSc) and polymyositis (PM), as in the prototype patient. These patients with anti-Kuantibodies appeared to have steroid-sensitive myositis, scleroderma limited to the forearms, and a relatively good prognosis.