Recent studies have demonstrated that most patients with T-cell acute lymphocytic leukemia (T-ALL) have activating mutations in NOTCH1. We sought to determine whether these mutations are also acquired in mouse models of T-ALL. We sequenced the heterodimerization domain and the PEST domain of Notch1 in our mouse model of TAL1-induced leukemia and found that 74% of the tumors harbor activating mutations in Notch1. Cell lines derived from these tumors undergo G(0)/G(1) arrest and apoptosis when treated with a gamma-secretase inhibitor. In addition, we found activating Notch1 mutations in 31% of thymic lymphomas that occur in mice deficient for various combinations of the H2AX, Tp53, and Rag2 genes. Thus, Notch1 mutations are often acquired as a part of the molecular pathogenesis of T-ALLs that develop in mice with known predisposing genetic alterations.