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Cell Biology Mechanical strain on cells can cause damage to the plasma membrane that must be repaired before extracellular Ca2+ influx reaches levels that trigger cell death. Horn et al. found that mitochondria mediated the repair of plasma membrane injuries in mouse muscle cells and human nonmuscle cells (see the Focus by Cooper). Ca2+ uptake by mitochondria initiated the generation of reactive oxygen species (ROS), which activated actin polymerization and wound closure. Quenching mitochondrially produced ROS in mouse muscle that was exercised ex vivo resulted in greater damage to myofibers and reduced muscle force. Sci. Signal. 10 , eaaj1978, eaao3795 (2017).