Mitochondria were isolated from Drosophila cells as described by Echalier [77]. For electron tomography, cells were fixed in 2% parafomaldehyde and 2.5% glutaraldehyde in 0.1 M sodium cacodylate, pH 7.4, post-fixed in 1% osmium tetroxide with 0.8% potassium ferrocyanide in sodium cacodylate buffer, stained with 2% uranyl acetate, dehydrated in a graded series of ethanol, and embedded in Durcupan ACM resin. Sections were cut with a thickness of approximately 250 nm from blocks exhibiting well-preserved ultrastructure. These sections were stained for 30 min in 2% aqueous uranyl acetate, followed by 30 min in lead salts. Fiducial cues consisting of 20-nm colloidal gold particles were deposited on both sides of the section.