Multiple Sclerosis (MS) is considered to be an autoimmune disease with unknown cause and with immune system dysregulation. Among environmental factors, viruses are most often connected with the etiology of MS.Human Endogenous Retroviruses (HERVs) constitute 5-8% of human genomic DNA and have been detected as transcripts and proteins in the central nervous system (CNS) and peripheral blood, frequently in the context of neuroinflammation. The HERV-Fc1, which belongs to the HERV-H/F family, has received our attention largely because of the genetic association with MS.The authors studied the expression of a capsid (Gag) protein of HERV-H/F origin by flow cytometry in PBMCs from healthy controls and from MS patients with non-active or active disease. There was a significant increase in HERV-H/F Gag expression in CD4+ (P