CD4 and neoplasm: During chronic viral infections and tumorigenesis, the exhaustion of virus- or tumor-specific CD4+ and CD8+ T cells is characterized by elevated levels of immune checkpoints, such as programmed death-1 (PD-1), programmed death ligand-1 (PD-L1), cytotoxic T-lymphocyte antigen 4 (CTLA4), T-cell immunoglobulin domain and mucin domain 3 (TIM-3), B- and T-lymphocyte attenuator (BTLA), lymphocyte activation gene 3 (LAG3), and cluster of differentiation 244 (CD244, also known as 2B4).