These results indicated that ATRA+HDACi treatment was not specific to SAHA, but is more generalizable, since a second‐generation HDAC inhibitor, romidepsin, also synergistically eradicated the tumor‐initiation property of human PDX xenografts in humanized NSG‐SGM3 mice (Figure 3A, right). The gene discussed is HDAC9; the disease is neoplasm.