In line with a recent study that reported the presence of a significant increase in TFH subsets in patients with infectious mononucleosis, a disease caused by EBV (17, 18), we demonstrate that EBV dUTPase and HHV-6A dUTPase–derived DCM, which contained high concentrations of activin A, also promoted the differentiation of naive CD4+ T cells into a TFH cell–like phenotype. This evidence concerns the gene CD4 and infectious mononucleosis.