The red-fluorescent CellTrace™ BODIPY® TR methyl ester is an excellent counterstain for cells and tissues that are expressing GFP. CellTrace™ BODIPY® TR methyl ester readily permeates cell membranes and selectively stains mitochondria and endomembranous organelles such as endoplasmic reticulum and the Golgi apparatus, but does not appear to localize in the plasma membrane. CellTrace™ BODIPY® TR methyl ester staining is retained after paraformaldehyde fixation.