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Background: Type 1 diabetes mellitus is a chronic autoimmune disease in which circulating antibodies to various islet-specific antigens including autoantibodies to glutamic acid decarboxylase (GADAb), antibodies reacting with an islet tyrosine phosphatase-related molecule termed as ICA512 (ICA512Ab), and insulin autoantibodies are frequently detected. These autoantibodies could be useful for presymptomatic diagnosis of type 1 diabetes mellitus, and tbeir presence suggest some patients with atypical diabetes mellitus that appears to be more prevalent in Asian than in western countries have autoimmune characteristics. ICA512Ab was discovered in 1992 and, when combined with GADAb, may increase the diagnostic sensitivity in autoimmune diabetes. In an attempt to study the autoimmune feature of atypical diabetes mellitus, we studied the prevalence of ICA512Ab using an in vitro transcription and translation method in the patients with insulin-dependent diabetes mellitus (IDDM), slowly progressive insulin- dependent diabetes mellitus (SPIDDM) and non-msulin-dependent diabetes mellitus (NIDDM), and compared it with that of GADAb. Methods: ICA512Ab were measured by a radioimmunoprecipitation method using in vitro transcribed and translated S-methionine-labeled ICA512. GADAb were measured using a commercial radioimmunoassay kit (RSR, United Kingdom). The subjects in this study consisted of 43 patients with IDDM, 32 with SPIDDM, and 40 witb NIDDM. Their mean age was 21.2+14.5 years, 50.1+17.1 years, 52.5+13.4 years, respectively. Results: The prevalence of ICA512Ab and GADAb in IDDM was 29 % and 51 %, respectively. That in SPIDDM was 9 % and 29 %; in NIDDM, 0 % and 2.5 %, respectively. When two antibodies were combined, 60 % of IDDM and 50 % of SPIDDM had the autoantibodies. When we analyzed the prevalence of autoantibodies according to the duration of diabetes, the prevalence of ICA 512Ab in patients tested within 4 years after the rliagnosis and more than 4 years after the diagnosis was 35 % and 19 %, respectively in IDDM. And also that of GADAb was 59 % and 38 %, respectively. In SPlDDM, the prevalence of ICA512Ab was 13 % and 7 %, respectively, while that of GADAb was 67 % and 14 % (p<0.05), respectively. In IDDM, ICA512Ab were more frequently detected in patients younger than 15 years ot age (45 %) than in older ones (14%) (p<0.05) while the prevalence of GADAb was not different according to the age (55 % vs 44 %). Conclusion: ICA512Ab are detected in some patients with autoimmune diabetes, while their prevalence is lower than that of GADAb. However, ICA512Ab, in combination with GADAb, increases the sensitivity ot autoantibody tests in autoimmune diabetes. Some of SPIDDM have an autoimmune etiology.