PDCD1 and Ewing sarcoma: Based on pre-clinical data demonstrating the beneficial effect of PD-1/PDL-1 blockade [50] and correlations between PDL-1 expression and clinical outcomes [51,52], clinical trials evaluating the effects of anti-PD1 antibodies are in progress in osteosarcoma and Ewing sarcoma (Pembrolizumab, NCT02301039 (available online: https://clinicaltrials.gov/ct2/show/NCT02301039) and Nivolumab, NCT02304458 (available online: https://clinicaltrials.gov/ct2/show/NCT02304458)).