Especially important, as a functional CD4+ T cell response represents a key factor dictating the outcome of viral infection, as viral clearance is associated with rapid recall of memory T-cell responses (Shoukry et al., 2003) which depends on critical CD4 T-cell help to maintain T-cell memory (Grakoui et al., 2003) and promote maturation of memory CD8+ responses (Zajac et al., 1998). This evidence concerns the gene CD4 and viral infectious disease.