CHO cells expressing A4V::YFP SOD1 fusion protein do not show evidence of increased membrane permeability. To assess whether aggregates of mutant SOD::YFP fusion proteins induce cell death, CHO cells were transiently transfected with expression vectors for SOD1 A4V::YFP (A and B) and WT::YFP (C). After 48 hours, the cells were incubated with dimeric ethidium bromide (EtBr) as described in Methods. Images were captured at 40× magnification (scale bar = 50 μm). Overall, there were few cells that showed evidence of increase membrane permeability with no obvious difference between cultures transfected with the 2 constructs or untransfected cells (D). As a positive control, untransfected cells were treated with staurosporine (+Str) to induce death and stained with dimeric ethidium bromide (D).