Provided that HERV-K18 superantigens are strictly dependent on Major Histocompatibility Complex class II for T cell activation, genetic epistasis between both loci is plausible and it seems from our data that a stronger association is found in the subgroup of MS patients carrying the HLA-DRB1*15∶01 risk allele (Table 1). This evidence concerns the gene HLA-DRB1 and myeloid sarcoma.