As a bioassay of NOTCH inhibition, we assayed the effect of small molecule NOTCH inhibitors on the proliferation of two cell lines immortalized from T-cell acute lymphoblastic leukemia (T-ALL) patients (Jurkat and KOPT-K1) and in the mantle cell lymphoma (MCL) line REC-1, all of which have activating mutations in the NOTCH1 heterodimerization and/or PEST domain common in hematologic malignancies20,21. This evidence concerns the gene NOTCH1 and acute lymphoblastic leukemia.