HeLa cells were grown to ~70 % confluence in 6-well dishes and transfected with 500 ng of either pExoS, pEGFP-CerADPr(E431D), or pEGFP-CerADPr. At 5 and 20 hr post-transfection (HPT), media was removed, cells were washed and stained for 5 min with trypan blue. Trypan blue was removed, cells were washed, and color images were obtained as described in Methods. Cytotoxicity was determined by counting the number of trypan blue positive HeLa cells / total number of HeLa cells per field. Five random fields were assayed from three biological replicates, and the results of this determination shown. 1-way ANOVA was performed on all columns ** p<0.01.