PDCD1 and cancer: The association of clinical responses to PD-1 pathway blockade with a high mutational burden [35], PD-L1 expression in tumors [36], and the presence of infiltrating CD8+ T cells [37], opposed to the lack of all these features in EwS [38, 39] (and Figure 3) argues against a major clinical impact of PD-L1/PD-1 blockade in this cancer.