Exhaustion is a fundamental event in many viral infections and PD-1 transcript levels are a marker of T-cell exhaustion,31 and increased PD-1 expression correlates with increased HSV-1 latency in TG.12,32,33 To compare PD-1 transcript levels with T-cell function in latently infected TG, mice were infected with KOS or D22 (ICP22 null) viruses as above. The gene discussed is PDCD1; the disease is viral infectious disease.