What: For patients with T cell acute lymphoblastic leukemia (T-ALL), a rare form of blood cancer that mainly affects children and young adults, drug resistance poses a major threat to a promising treatment option currently in clinical trials. About half of patients with T-ALL have mutations in NOTCH1, but drugs that target this gene have so far produced only short-lived effects: at first, the cancer seems to respond, but in a short period of time, T-ALL returns.