The MitoTracker staining likely overestimates the uptake because cancer cell mitochondria fuse with the mitochondria of recipient HPFs, as seen after transplantation of MitoTracker green-labeled or Su9–RFP-expressing mitochondria from A431 cells into HPFs prestained with MitoTracker red or expressing TOM20–GFP, respectively (Fig. 4c). This evidence concerns the gene UQCRFS1 and cancer.