GDF6 and Ewing sarcoma: Small interfering RNA (siRNA)-mediated silencing of GDF6 strongly inhibited the proliferation of all 12 Ewing sarcoma cell lines tested (A673, TC71, EW8, ES1, ES2, ES4, SK-N-MC, SK-ES-1, CHLA-258, SK-NEP-1, SK-PNET-Li, and CHLA-352; Figures 2A and S1A).