TMRM is a dynamic mitochondrial stain. There is no covalent attachment of TMRM to the mitochondria and because of this, it can move in and out of the mitochondria dependent upon changes in the membrane potential. The MitoTracker dyes on the other hand possess a reactive chloromethyl group that allows for their covalent attachment to mitochondria. As a result, once they accumulate within mitochondria that have a normal membrane potential, they are retained.
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