It is recognised that a plethora of immune regulatory factors in the tumour microenvironment (TME) contribute to the progression of cancers and limit their response to treatment.1–3 An important class of inhibitory factors, designated immune checkpoints, have been associated with sustained tumour responses in a variety of cancers.4,5 The programmed cell death 1 (PD-1) receptor has emerged as a dominant negative regulator of anti-tumour effector function. Here, PDCD1 is linked to cancer.