When using a fluorescein dye labeled probe during the FISH procedure, which counterstain should be applied?

In fluorescence in situ hybridization (FISH), when using a fluorescein dye labeled probe, the appropriate counterstain to apply is propidium iodide. Propidium iodide is a DNA intercalating agent that stains the nuclei of cells and emits red fluorescence upon binding to DNA. This creates a contrast between the green fluorescence from the fluorescein-labeled probe and the red fluorescence from the propidium iodide, allowing for clear visualization of both the probe and the cellular structures.

Using propidium iodide as a counterstain enhances the visibility of the hybridization signals, making it easier to distinguish between the target regions of interest and the rest of the cell. The combination of the fluorescein signal with the propidium iodide counterstain is effective in identifying specific genomic sequences while simultaneously providing information about the overall cellular context.

Other counterstains may not provide the same level of contrast or may interfere with the fluorescein signal, which is why propidium iodide is favored in this scenario.