The mRNA expressions of PD-1 and PD-L1 in tumor lesions observed in kidneys of chickens experimentally infected with MDV-1. A total of 9 tumor samples were collected at 21, 28, and 35 d.p.i. (n = 2, 3, and 4, respectively). Two kidney samples were also collected from uninfected chickens and analyzed as controls. The expressions of PD-1 (A), PD-L1 (B), IFN-γ (C), and meq (D) mRNA were determined by real-time RT-PCR. The concentration of each mRNA was normalized to that of β-actin mRNA. The extent of gene expression (−fold) was calculated by dividing the value of each sample by that of the value of each control (A-D). Error bars represent standard deviations. The significant differences were determined by Student’s t-test (**P < 0.01).