Exposing rag2:hMYC-ER larvae to 4HT beginning at 5 dpf caused 100% T-ALL penetrance by five weeks of age, but many cancers regressed upon 4HT withdrawal, suggesting some ‘leukemias’ were not stably transformed, but rather lymphoproliferations that relied upon very high MYC activity. This evidence concerns the gene MYC and acute lymphoblastic leukemia.