SP2577, 2, originally derived from SP2509, 3, is another LSD1 inhibitor in clinical trials for Ewing sarcoma (Fig. 1A).9,10 SP2509 and SP2577 are reported to be reversible non-competitive inhibitors of LSD1, differing to the covalent LSD1 inhibition of TCP. Here, KDM1A is linked to Ewing sarcoma.