The balance between the recruitment and functional state of CD8+ cytotoxic T cells (Tc), CD4+ conventional T cells (Tconv), and CD4+CD25+FoxP3+ regulatory T cells (Treg) is responsible for maintaining tolerance to self, while also responding to pathogenic challenges arising from foreign bacteria and viruses or endogenous stimuli in the form of cancer. The gene discussed is CD4; the disease is cancer.