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The Kearns-Sayre syndrome (KSS) is a distinctive type of progressive external ophthalmoplegia, characterized by pigmentary degeneration of the retina, heart block, elevated concentration of cerebrospinal fluid protein, and abnormal muscle mitochondria. Previously described cases have all been sporadic. Consequently, viral infections and autoimmune disorders have been proposed as etiologies. The occurrence of KSS in two brothers suggests that genetic factors may play a role in the pathogenesis of some cases.