By using TCR sequences as a molecular barcode to link clonally related T cells in the tumor and blood, we identified a subpopulation of circulating predicted-reactive CD8+ T cells resembling KIR+CD8+ Tregs, which have been shown to specifically kill pro-inflammatory, autoreactive T cells in patients with autoimmune diseases and viral infections. The gene discussed is KIR3DL1; the disease is viral infectious disease.