Depletion of CD8 T cells resulted in increased viral titers of Marek’s disease virus (MDV) in CD4 T cells from CVI988-immunized chickens and more tumor development in monovalent (SB-1 or HVT) and/or bivalent (SB-1 + HVT)-immunized chickens after MDV challenge (Morimura et al., 1998, 1999; Umthong et al., 2020). The gene discussed is CD8A; the disease is neoplasm.