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We review the newest advances related to seeking the pathogenic mechanism(s) of sporadic inclusion-body myositis (s-IBM) and present the pathologic diagnostic criteria of s-IBM. We discuss the possible pathogenic role of several themes, such as 1) increased amyloid-[beta] precursor protein (A[beta]PP) and of its fragment A[beta]; 2) phosphorylation of tau protein; 3) oxidative stress; 4) abnormal a) signal-transduction, b) transcription, and c) RNA accumulation; 5) "junctionalization" and "myogenous" denervation; and 6) lymphocytic inflammation. Evidence is provided supporting our hypothesis that overexpression of A[beta]PP within the aging muscle fibers is an early upstream event causing the subsequent pathogenic cascade. The remarkable pathologic similarities between s-IBM muscle and Alzheimer disease (AD) brain are discussed, and the possible cause and significance are addressed.

(C) 2001 American Association of Neuropathologists, Inc