(A) Relative immunofluorescence of OPA1 in INS1 cell mitochondria correlates to their fusion capacity. Cells (n=6) were photoactivated (∼10-15% of cell mitochondrial mass) and then returned to the incubator for 2 h before fixation. Based on their ability to share and thereby dilute the photoconverted mtPA‐GFP, mitochondria were classified as fused (n=54) in which GFP FI was lower than 2 times the average and non‐fused (n=10) in which GFP FI was at least 200% above the average (in the same manner as described for Figure 1). In each cell, OPA1 FI of non‐fused mitochondria was normalized to that of mitochondria that had diluted mtPA‐GFP through fusion. OPA1 FI is reduced in non‐fusing mitochondria (P=0.003). A set of FI‐coded images of mtPA‐GFP, mtDsRed and OPA1 immunostaining are shown. Scale bar, 2 μm. 