Subsequent protein microarray-based testing of recombinantly expressed CSF-derived antibodies against MS-associated viruses, a sequence analysis, affinity measurements, and the crystal structure of the EBNA1-peptide epitope in complex with the autoreactive Fab fragment enabled tracking of the development of the naive EBNA1-restricted antibody to a mature EBNA1-GlialCAM cross-reactive antibody. The gene discussed is HEPACAM; the disease is myeloid sarcoma.