In cancer immunotherapy, TCR profiling offers predictive insights; high baseline tumor clonality frequently correlates with response to anti-programmed cell death protein 1/programmed death-ligand 1 inhibitors, while greater peripheral diversity may predict benefit from anti-cytotoxic T-lymphocyte-associated protein 4 (anti-CTLA-4) therapy. The gene discussed is CTLA4; the disease is neoplasm.