The present invention concerns a DNA-replication-associated (Rep) protein comprising an amino acid sequence as depicted in SEQ ID NO: 11 or 12; (b) a fragment of SEQ ID NOs:11 or 12 which is capable of binding an anti-Rep antibody specific for a protein having the amino acid sequence of SEQ ID NOs: 11 or 12; or (c) an amino acid sequence having a 90% or more homology to the amino acid sequence of (a) or (b) and is capable of binding an anti-Rep antibody specific for a protein having an amino acid sequence of SEQ ID NOs:11 or 12. The present invention further concerns a method of diagnosing MS or a predisposition for MS and a kit for use in such methods.