Mathematical analysis of murine T cell responses to viral infection has shown that CD4+ T cells responding to an immunodominant epitope in lymphocytic choriomeningitis virus (LCMV) begin to divide between 48 and 72 h after stimulation and have a doubling time of 11 h in vivo (De Boer et al., 2003). The gene discussed is CD4; the disease is viral infectious disease.