In addition to possibly promoting regulatory T cells, TGF-β could also negatively affect cytotoxic T cell differentiation, as a recent study on oral squamous cell carcinoma illustrates the significance of TGF-β in attenuating the cell cycle of CTL, inhibiting their proliferation during effector phase as well as their differentiation into TEM, promoting apoptosis induction, and ultimately causing a decrease in the CD8+ T cell/Treg ratio (64). The gene discussed is TGFB1; the disease is oral cavity squamous cell carcinoma.