PDCD1 and metastatic neoplasm: Additionally, studies have shown that up to 97% of LM-NEN are infiltrated with T-cells, immunosuppressive regulatory T-cells (Tregs) and have surface expression of immune exhaustion markers (programmed cell death protein 1, PD-1 and programmed death ligand 1, PD-L1) (10, 11) making these tumours potential candidates for immune checkpoint inhibitor therapy, contrary to non-metastatic disease in which checkpoint inhibitors have had marginal success (12).