Multiple sclerosis (MS) is still of unknown origin and may involve autoimmune, genetic and viral components in a pathogenic sequence whose relative importance is yet to be determined. A peptide, isolated from the cerebrospinal fluid of MS patients, is similar to a fragment of the pol protein reverse transcriptase (RT) of the newly reported MSRV retrovirus. The 700 amino acid sequence of MSRV-RT is closely related to a novel human retroviral-like sequences. We also identified a gag-like sequence upstream of this human genomic RT-like sequence, which allowed us to identify altogether 4,000 nucleotides, possibly coding for an endogenous retroviruses. Homologous sequences found in other locations in the human genome seem to characterize a new family of retroviral endogenous sequences, which may be of relevance to multiple sclerosis.