Correspondingly, addressing the localization of NRB could help characterize the molecular mechanisms underlying the species- and tissue-selective actions of NRB; the original primary objective of this study was therefore to develop a fluorescent derivative of NRB (NRB-AF12, see Figure 1B for structure) in order to compare its subcellular distribution in NRB-sensitive and NRB-insensitive cells. However, since preliminary results indicated a strong overlapping fluorescent distribution between NRB-AF12 and that of ER-Tracker® (ER-Tr), a commercially available fluorescent probe widely used as a selective marker for endoplasmic reticulum (ER), we extended our goals to include a comparison of the staining profiles of the two dyes. The results indicate that NRB-AF12 and ER-Tr show a near identical pattern of fluorescence distribution in both NRB-sensitive and NRB-insensitive cells, and that this fluorescence profile differs between the two cell types. Furthermore, we found that both dyes label not only ER (as would be expected with ER-Tr), but also Golgi apparatus and mitochondria. In addition, unlike ER-Tr, NRB-AF12 was also able to label lysosomes and endosomes.