To test whether Ddb1 was required for clonal expansion and function of effector CD4+ T cells during acute viral infection, we crossed Ddb1fl/fl; OX40-cre mice with SMARTA TCR transgenic mice (29), which have transgenic expression of a T-cell antigen receptor specific for LCMV GP61-80, presented by the major histocompatibility complex (MHC) class II molecule I-Ab, to generate Ddb1-TaKO SMARTA mice. This evidence concerns the gene CD4 and viral infectious disease.