PDCD1 and neoplasm: PD-1 has at least 2 ligands (PD-L1 and PD-L2), of which PD-L1 is expressed on a variety of malignant tumor cells.[16] Tumor cells can manipulate their microenvironment to be conducive to tumor growth by activating the PD-1/PD-L1 signaling pathway, which inhibits T-cell activation.