CD4 and viral infectious disease: Functionally, CLOCK<sup>Δ19</sup> produces β-catenin stabilization in T cells, pronounced expansion of RORγt<sup>+</sup> CD4<sup>+</sup> T cells and Treg cells, impaired Treg suppression of Th17 responses, heightened Th17 responses, and reduced IFN-γ production during viral infection.