Thomas Hunt Morgan first described a mutant Drosophila melanogaster strain with “notched” wings in 1917. The gene responsible for this phenotype was discovered many years later and named Notch (1). The human Notch1 homolog, also referred to as TAN1, was identified in T-cell acute lymphoblastic leukemia (T-ALL) patients with the t(7;9)(q34;q34.3) chromosomal translocation (2). The mammalian Notch proteins are heterodimeric transmembrane receptors that control cell proliferation, apoptosis, and cell fate during the development of diverse cellular lineages (3). In adults, Notch signaling regulates stem cell maintenance, binary cell-fate decisions such as B- versus T-lineage differentiation, and differentiation of self-renewing organs (4).