Human NOTCH1 was cloned as a chromosome 9 gene, translocated to the T-cell beta receptor (TCBR) promoter on chromosome 7 in T-cell acute lymphoblastic leukemia (T-ALL) (Ellisen et al. 1991). The translocated gene was found to be homologous to Drosophila Notch, and was initially named TAN-1 (translocation-associated Notch homolog). Although the translocation t(7;9)(q34;q34.3) is present in a small percentage of T-ALL patients, the mutant protein is highly oncogenic and its overexpression causes T-ALL-like illness in mice (Pear et al. 1996).