293T cells were maintained in DMEM supplemented with 10% fetal calf serum. Confluent 293T cells were trypsinized, split 1:10 and seeded onto 10-cm dish plates 24 hours before transfection. For each construct, 293T cells were transfected with 20 mg of plasmid DNA by the calcium phosphate precipitation method (18) , with the addition of 50 mM chloroquine to enhance transfection efficiency.