Abstract: The present invention relates to compositions and methods that may be used to diagnose and treat cancer, particularly T-cell leukemia. According to one preferred embodiment of the present invention, methods are provided for determining whether reducing or blocking NOTCH-1 activation will be effective to treat, prevent, or ameliorate the effects of a cancer in a patient, including T-cell leukemia, myeloleukemia, neuroblastoma, breast cancer, and ovarian cancer. The methods generally include determining if the patient harbors one or more mutations in a PTEN coding region. In particular, the methods may be used to determine whether reducing or blocking NOTCH-1 activation, with one or more ?-secretase inhibitors, will be effective to treat, prevent, or ameliorate the effects of a cancer in a patient.