Mitochondria frequently fuse and divide (Bereiter-Hahn & Voth 1994); these processes affect morphology and are important for normal mitochondrial functions such as respiration, development and apoptosis. Mitofusins (MFNs) are mitochondrial GTPases that mediate mitochondrial outer membrane fusion. Mammals have two mitofusins; Mfn1-null or Mfn2-null mouse embryonic fibroblast cells show predominantly fragmented mitochondria and have greatly reduced mitochondrial fusion in vivo (Chen et al. 2003, 2005). MFNs acts in trans to bring mitochondria into close proximity prior to fusion (Koshiba et al. 2004). They also tether the endoplasmic reticulum (ER) to mitochondria, cross-linking MFNs expressed on the mitochondrial outer membrane and ER membrane (de Brito & Scorrano 2008).