Immune co-inhibitory molecules, such as programmed cell death protein-1 (PD-1/PD-L1) and cytotoxic T-lymphocyte antigen 4 (CTLA-4), are also expressed in MTC tumour cells, thus immune checkpoint inhibitors could also be a possible therapeutic option in progressive MTC. The gene discussed is PDCD1; the disease is medullary thyroid gland carcinoma.