NR2E3 and Ewing sarcoma: Clofarabine is a reversible inhibitor of RNR and has a half-life of ~5-7 hours in vivo so we next asked whether more prolonged, sustained inhibition of RNR, similar to that achieved in the in vitro assays (Figure 1), is required to induce apoptosis and toxicity in Ewing sarcoma cells [41, 50, 51].