Synergistic induction of cell death by LSD1 and HDAC inhibitors has previously been described for glioblastoma,32, 33 AML34 and breast cancer.35, 36 Our study not only reveals a synergism of concomitant LSD1 and HDAC inhibition in RMS, but – more importantly – elucidates the molecular mechanisms underlying this synergistic induction of cell death. This evidence concerns the gene HDAC9 and breast cancer.