The invention relates to: a nucleotide fragment of an MSRV-1 LTR-RU5 region, comprising a nucleotide sequence encoding the expression of a protein, wherein the protein comprises a peptide sequence selected from the group consisting of: SEQ ID NO: 2, SEQ ID NO: 3, and SEQ ID NO: 4; complementary nucleotide fragments; probes and primers that hybridize to the fragment; proteins encoded by the fragment; antibodies directed against the proteins encoded by the fragment; and processes for detecting the presence of MSRV-1 using a probe or an antibody of the invention.