PD-1 has been widely accepted as a marker for both T cell activation and exhaustion, and it was demonstrated by Steven Rosernberg’s group with tumor and blood samples from cancer patients that PD-1+ CD8+ T cells represent patient-specific, tumor-reactive CD8+ T cell repertoire marked by an ability to recognize a variety of tumor antigens (Gros et al., 2016). The gene discussed is CD8A; the disease is cancer.