In this issue of the JCI, Hansen and colleagues report on their tracking of viral T antigen–reactive (T-Ag–reactive) CD8+ T cells in the peripheral blood of 26 patients with MCC undergoing anti–programmed cell death protein-1 (anti–PD-1) therapy in a clinical trial (ClinicalTrials.gov NCT02267603) (4). The gene discussed is CD8A; the disease is Merkel cell skin cancer.