rMDV encoding L-Meq (vRB-1B_L-Meq) exhibited higher mortality and tumor incidence than rMDV encoding Meq (vRB-1B_Meq), and vRB-1B_L-Meq infection increased the proportion of CD4+ T cells, which are targets for transformation by MDV, in the early post-infection phase, compared with vRB-1B_Meq. The gene discussed is CD4; the disease is neoplasm.