To investigate whether IL-27 intrinsically regulated the number of antiviral CD4 T cells upon infection, we generated WT:Il27ra-/- chimeric mice by reconstituting sublethally irradiated recipients with a 1 to 1 ratio of WT and Il27ra-/- bone marrow cells and tracked WT versus Il-27ra-/- CD4 T cells at different times post infection based on congenic marker expression (S1A–S1C Fig). The gene discussed is CD4; the disease is infection.