LYN and lymphopenia: Unexpectedly, aged G‐CSF−/− mice exhibited an expansion of CD4+ T cells, and contrary to the well‐established CD4+ T cell lymphopenia observed in aged Lyn−/− mice, Lyn−/−G‐CSF−/− mice had normal numbers of CD4+ T cells, comparable to C57BL/6 controls (Figure 4A,B).