CD99 and Ewing sarcoma: Agents being investigated in the relapsed Ewing’s sarcoma setting include epigenetic therapies (e.g., inhibitors of lysine-specific demethylase 1 (LSD1), histone deacetylases, and bromodomain-containing proteins), inhibitors of various downstream components of the EWS-FLI1 transcriptional network (TKI-216), agents that bind to DNA and disrupt processing of DNA by multiple pathways (e.g., plicamycin and trabectedin), CD99 targeting agents (clofarabine/cladribine and anti-CD99 antibodies), an anti-insulin-like growth factor receptor antibody (Ganitumab), and others [3].