Specifically, PD-1 expression in splenic CD4+ T cells barely increased during the first three weeks post-infection and then rapidly increased (average fold-increase of 4.1 in frequency, 5.6 in total number, and 2 in MFI) and reached a plateau at five weeks post-infection, remaining at a high level thereafter (Fig 2B and 2C and S1 Fig). The gene discussed is CD4; the disease is infection.