Differences in the immune response of different mouse strains and substrains to injuries, cancers and infections including IAV are well-appreciated (38), but this is the first time, to our knowledge, that the kinetics and subset distribution of the complete endogenous CD4 T cell response within the lungs during IAV has been directly compared in BALB/c and C57Bl/6 mice. The gene discussed is CD4; the disease is infection.