Resistance mutations (primary and/or acquired) that could help avoid non-beneficial therapies were also observed and included SDH loss, PDGFRAD842V, and KIT mutations specifically associated with imatinib resistance in GIST, ESR1 mutations potentially associated with anti-estrogen resistance in endometrial stromal sarcoma, inactivating TP53 mutations associated with MDM2 inhibitor resistance, and RB1 deletion associated with resistance to CDK4 inhibitors in dedifferentiated liposarcoma59–65. The gene discussed is RB1; the disease is endometrioid stromal sarcoma.