It suppresses T-cell function by recruitment of phosphatases to an immunoreceptor tyrosine-based switch motif thereby countering positive signaling by the TCR and CD28.11 PD-1 also inhibits T-cell function by increasing expression of transcription factors such as basic leucine zipper transcriptional factor ATF-like, which inhibits T-cell effector function.12 Its principal ligand is programmed death-ligand 1 (PD-L1), which is expressed by tumors and tumor-infiltrating immune cells. Here, PDCD1 is linked to neoplasm.