Labeling of organelles for microscopy is achieved generally by specific dyes that either accumulate in a cellular compartment such as cyanine dyes in mitochondria or are only fluorescent under specific conditions such as the low pH in the lysosome. Here we demonstrate that Cy5—a fluorescent molecule that does not enter cells by itself—can be loaded into cells by attaching a short oligonucleotide. This very inexpensive labeling procedure can be done in the presence of serum. Therefore, very sensitive cell types should also be amenable to this procedure, and longer observations can be achieved compared to other commercially available dyes as the labeling reagent does not need to be washed out. This also points to the pitfall of using fluorescently labeled oligonucleotides for live cell imaging where the oligonucleotide is supposed to detect a specific target sequence in its subcellular distribution.
