To investigate whether T-bet is required for optimal TFH cell responses during acute viral infection, we generated a CD4cre-Tbx21fl/fl strain of mice (called Tbx21−/− mice here) by crossing transgenic CD4cre mice with Tbx21fl/fl mice to selectively knock out the Tbx21 gene (encoding the T-bet protein) in T cells. Here, TBX21 is linked to viral infectious disease.