CTLA4 and neoplasm: cis-CD80:PD-L1 interaction also inhibits CTLA-4:CD80 binding by an avidity effect rather than by directly blocking the CTLA-4 binding site on CD8032,40, Fig. 1D. Although cis-CD80:PD-L1 disrupts inhibitory functions of both CTLA-4 and PD-1, tumour cells often have low CD80 expression and this mechanism is not effective41.