These tumour-intrinsic and tumour-extrinsic factors include mutations in key effector pathways, high levels of PD-L1 on tumour cells (and immune cells), high levels of alternate immune checkpoints or co-inhibitory receptors on T cells (e.g., PD-1, CTLA-4), high levels of immune suppressive cytokines or metabolites, and associated recruitment of immune suppressive cells (e.g., myeloid-derived suppressor cells (MDSCs) and regulatory T cells (Tregs)) (O'Donnell et al, 2017). Here, CTLA4 is linked to neoplasm.