As neoantigen-specific TIL are typically inhibited in situ by other factors beyond the PD-1/PD-L1 axis, a more thorough understanding of both the phenotypic nature of TIL and the stimulatory and inhibitory interactions they undergo within the tumour microenvironment will inform future therapies targeted at maximizing their anti-tumour function. This evidence concerns the gene PDCD1 and neoplasm.