Reduced CD5 levels on T cells within the 4T1 tumor microenvironment may be evidence of selection of T cells with naturally-occurring low CD5 for enhanced activation and proliferation within tumors: such selection would promote interaction between T cells and tumor cells, enhance the TCR signaling response, and result in increased activation as shown by increased CD69, PD-1 and CTLA-4 expression. The gene discussed is CTLA4; the disease is neoplasm.