PDCD1 and endometrial cancer: While preliminary evidence exists that tumor cell surface PD-L1 expression correlates with the likelihood of response to PD-1 pathway inhibition [66], the best argument for the use of checkpoint inhibitors in select endometrial cancer cases was recently put forth by a phase 2 trial of pembrolizumab, a humanized monoclonal antibody to the PD-1 receptor, in patients with mismatch repair- (MMR-) deficient tumors [67].