HDAC9 and cancer: Compared with cancer PDOs, normal gastric organoids tended to be more responsive to MEK1/2 inhibition with trametinib (area under curve (AUC)rel: 0.438 (normal) vs. 0.614 (PDOs)), while they tended to be less responsive to HDAC inhibitors (AUCrel normal vs. PDOs: panobinostat: 0.582 vs. 0.524, entinostat: 0.581 vs. 0.563, tacedinaline: 0.628 vs. 0.598, and vorinostat: 0.576 vs. 0.554) (Table EV3).