The increase in intracellular copper ion concentration may increase PD-L1 expression by up-regulating signal transducer and activator of transcription 3-dependent transcription or inhibiting ubiquitin-proteasome system-mediated PD-L1 degradation, thereby enhancing tumor susceptibility to therapy targeting the programmed cell death 1 (PD-1)/PD-L1 axis 61. Here, PDCD1 is linked to neoplasm.