CD8+ tumor infiltrating T cells (TIL) lack effector-phase functions due to defective proximal TCR-mediated signaling shown to result from inactivation of p56lck kinase. We identify a novel interacting partner for p56lck in nonlytic TIL, Protocadherin-18 (pcdh18), and show that pcdh18 is transcribed upon in vitro or in vivo activation of CD8+ central memory T cells (CD44+CD62LhiCD127+) coincident with conversion into effector memory cells (CD44+CD62LloCD127+). Expression of pcdh18 in primary CD8+ effector cells induces the phenotype of nonlytic TIL: defective; proximal TCR signaling, cytokine secretion, and cytolysis; and enhanced AICD. pcdh18 contains a motif (centered at Y505) shared with src kinases (QGQYQP) which is required for the inhibitory phenotype. Thus, pcdh18 is a novel marker of CD8+ effector memory T cells expressed upon cell activation that can function as a negative regulator by restricting the effector phase.