To determine the level of activation of CD5high and CD5-/low tumor-infiltrating T cells, the level of the activation markers including CD69 (26), PD-1 (27), and CTLA-4 (28) in both CD4+ and CD8+ T cell subpopulations in T cells isolated from 4T1 tumors excised from mice at 21 days following subcutaneous implantation of tumor cells was determined. The gene discussed is CD5; the disease is neoplasm.