NOTCH1 encodes a member of the NOTCH family of proteins, which is a group of transmembrane receptors involved in the Notch signaling pathway. Notch signaling regulates cell fate decisions during development, and plays a crucial role in T cell development. Activating mutations of NOTCH1 including missense mutations and inframe inserstions/deletions in the heterodimerization(HD) domains either alone or together with missense, nonsense or frameshift (in/del) mutations in the C terminal PEST domain have been described in approximately 50% of cases of T-ALL. The HD domain or PEST domain mutations may occur together in cis (on the same allele) in ALL. NOTCH1 mutations are very rare in AML. However, NOTCH1 mutations are present in about 27% patients with T-myeloid mixed-phenotypeacute leukemia. The potential utility of therapeutic targeting of activating NOTCH1 mutations in these diseases remains to be elucidated.