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I ntestinal colonic crypts are derived from a stem cell population located at the base of each crypt. A new analysis of mitochondrial function and of the rates of mitochondrial DNA (mtDNA) mutation in individual crypts shows that mtDNA mutations arise in stem cells — and at a surprisingly high frequency. Because crypts turn over extremely rapidly (about once per week), somatic mtDNA mutations can “take over the system” and even become homoplasmic, in a manner similar to what has been shown to occur in tumors.