Tetramethylrhodamine, ethyl ester (TMRE) is a cell-permeant, cationic, red-orange fluorescent dye that is readily sequestered by active mitochondria. The dyes do not form aggregates in cell membranes and have minimal interaction with membrane proteins. In addition to staining mitochondria for imaging purposes, TMRE is used for quantitative measurements of mitochondria membrane potential using Nernst equation.