To discriminate between the type I IFN responses induced in active SLE patients from those induced in ND after viral infection, we compared the gene expression profiles of sorted peripheral blood CD4+ T cells, CD16− inflammatory monocytes and CD16+ resident monocytes from 8 SLE patients and 4 ND 7 days after yellow fever vaccination. The gene discussed is CD4; the disease is systemic lupus erythematosus.