This is one of five imaging datasets to compare mtDNA mutant cell lines with isogenic wild-type cell lines by mtFociCounter. Images are Spinning Disk Confocal Images of unsorted 3t3 NIH mouse fibroblasts stably expressing mitochondriallytargeted dsRed, immunofluorescence against dsDNA (mitochondrial nucleoids) and AlexaFluor 647, and stained nucleiwith Hoechst. Samples from the same day are 13mm coverslips processed in parallel (seeding of cells, fixation, immunofluorescence, microscopy) and mounted on the same glass slide (#1 on left, #2 centre, #3 right). This dataset contains data from WT cells and COXII loss of function mutations from the following acquisition dates: 20221022 20221026 20221029 For further details, please do not hesitate to contact the author, timorey AT hotmail DOT com. A preprint with experimental details and analysis will be available soon.