Besides the traditional functions of Th1 and Th2 cells in priming and helping tumor-specific CD8+ T cells and B cells, recent discovery of Th17 and Treg cells has not only resulted in an explosion of cancer immunological research but also markedly changed our conventional thinking of the role of CD4+ T cells in the pathogenesis of cancer development [17–25]. The gene discussed is CD8A; the disease is cancer.