Although the etiology of multiple sclerosis (MS) is unknown, there is compelling evidence that its pathogenesis is mediated through the immune system. Molecular mimicry, i.e., crossreactivity between self-antigens and viral proteins, has been implicated in the initiation of autoimmunity and MS. Based on homology to human T cell lymphotropic virus type I (HTLV-I) a novel human retrotransposon was cloned and found to constitute an integral part of the coding sequence of the human transaldolase gene (TAL-H). TAL-H is a key enzyme of the nonoxidative pentose phosphate pathway (PPP) providing ribose-5-phosphate for nucleic acid synthesis and NADPH for lipid biosynthesis. Another fundamental function of the PPP is to maintain glutathione at a reduced state and, consequently, to protect sulfhydryl groups and cellular integrity from oxygen radicals. Immunohistochemical analyses of human brain sections and primary murine brain cell cultures demonstrated that TAL is expressed selectively in oligodendrocytes at high levels, possibly linked to production of large amounts of lipids as a major component of myelin, and to the protection of the vast network of myelin sheaths from oxygen radicals. High-affinity autoantibodies to recombinant TAL-H were detected in serum (25/87) and cerebrospinal fluid (15/20) of patients with MS. By contrast, TAL-H antibodies were absent in 145 normal individuals and patients with other autoimmune and neurological diseases. In addition, recombinant TAL-H stimulated proliferation and caused aggregate formation of peripheral blood lymphocytes from patients with MS. Remarkable amino acid sequence homologies were noted between TAL-H and core proteins of human retroviruses. Presence of crossreactive antigenic epitopes between recombinant TAL-H and HTLV-I/human immunodeficiency virus type 1 (HIV-1) gas proteins was demonstrated by Western blot analysis. The results suggest that molecular mimicry between viral core proteins and TAL-H may play a role in breaking immunological tolerance and leading to a selective destruction of oligodendrocytes in MS.