We first compared phototoxicity of KillerRed-Rab7 with that of KillerRed localized to mitochondria (KillerRed-mito) or plasma membrane (KillerRed-mem), which were previously shown to efficiently mediate light-induced cell death.11,13,16–18,20 HeLa cells were transiently transfected with KillerRed-Rab7, KillerRed-mito, KillerRed-mem, or KillerRed-C (the latter as a negative control with low phototoxicity) and mixed with HeLa stable cell line expressing green fluorescent protein TurboGFP. Culture dishes with mixed populations of cells were illuminated with green LED array (530 nm, for 1 h) or kept in the dark (control cells) and left to grow for 24 h. Then several random fields of view were analyzed by fluorescence microscopy for each dish to compare number of KillerRed- and TurboGFP-expressing cells. We found that green light illumination practically did not affect cells expressing free cytoplasmic KillerRed-C but resulted in a dramatic decrease of the KillerRed-expressing cells numbers for KillerRed-Rab7 (-fold, , Fig. 1), KillerRed-mito (-fold, ), and KillerRed-mem (-fold, ). As the observed differences between KillerRed-Rab7, KillerRed-mito, and KillerRed-mem were not statistically significant, we concluded that lysosome-associated KillerRed ensures efficient light-induced cell death similar to previously reported localizations to mitochondria and plasma membrane.