The MPVPSAPST-peptide is the amino-terminal part of a small hypothetical protein of 29 amino acids encoded by the complementary DNA strand of the HERV-WE1 env gene which is conserved in all homologous HERV-W sequences in the human genome. Gene transcripts of 21 of 25 open reading frames with an initiating start codon have been found in association with MS according to genetic data bank entries. Moreover, several other HERVs (-H, -K, -L) exhibit this homology. As these HERV peptides are all self-antigens, they could serve as targets, but not as inducers, of the postulated MS-protective immune response. The main HLA class I molecule A*0201 for the presentation of the peptide, with a frequency of about 30% in a European population, was shown to be associated with a significantly reduced MS risk (OR = 0.52, P = 0.0015) [53] .