Fis1, a 17kD integral membrane protein, is a novel member of mitochondrial fission machinery. It consists of a central leucine-zipper domain, a coiled-coil region, a tetratricopeptide repeat domain and a C-terminal transmembrane domain. It is a human orthologue of Fis1p (S.cerevisiae) and is localized in the outer mitochondrial membrane. Fis1 acts as a pro-apoptotic factor and plays a vital role in the regulation of mitochondrial fission. It interacts with DLP1 and Mdv1p and facilitates Dnm1p recruitment to the functional fission complex, thereby inducing apoptosis. Role of Fis1 has also been implicated in growth and division of peroxisomes. Loss of Fis1 inhibits apoptosis and results in elongation of mammalian mitochondria.