Amyloid precursor protein (APP) has been implicated in the pathogenesis of Alzheimer disease, and the accumulation of APP products ultimately leads to the familiar histopathological and clinical manifestations associated with this most common form of dementia. A protein that has been shown to promote APP accumulation is beta-secretase (beta-site APP cleaving enzyme 1, or BACE1), which is increased in the cerebrospinal fluid in those affected with Alzheimer disease. Through in vivo studies using APP transgenic mice, we demonstrated that decreasing the expression of BACE1 via lentiviral vector delivery of BACE1 siRNA has the potential for significantly reducing the cleavage of APP, accumulation of these products, and consequent neurodegeneration. As such, lentiviral-expressed siRNA against BACE1 is a therapeutic possibility in the treatment of Alzheimer disease. We detail the use of lentivirus-expressed siRNA as a method to ameliorate Alzheimer disease neuropathology in APP transgenic mice.