PDCD1 and neoplasm: Expression of PD-1 ligand (PD-L1) by tumor lines is being investigated as a predictive biomarker with distinct genetic and morphological characteristics (e.g., increased aggressiveness) [160], with preliminary data suggesting that tumors lacking PD-L1 expression are less likely to respond to PD-1 inhibition compared to tumors expressing PD-L1 [161].