PDCD1 and neoplasm: The first evidence of a role for PD-1 in tumour immune evasion came from a study showing that transgenic PD-L1 expression by a tumour cell line rendered it less susceptible to the specific TCR-mediated lysis by cytotoxic T cells in vitro, and markedly enhanced its tumorigenesis and invasiveness in vivo in the syngeneic hosts as compared to the parental tumour cells [37].