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Cover  Mitochondria in living cells are dynamicfrequently fusing, dividing, and changing shape. Indeed, the steady-state copy number and shape of mitochondria varies dramatically in different cell types, ranging from multiple spherical organelles to a single-copy branched structure. We now appreciate that the regulation of mitochondrial morphology is important for cell function. For example, mitochondrial fusion is required for sperm development in flies, and fission plays a fundamental role in the process of apoptotic cell death (Hales and Fuller (1997) Cell 90, 121-129; Frank et al. (2001) Dev. Cell 1, 515-525). The model organism, S. cerevisiae, has played a central role in unraveling the molecular mechanisms underlying the establishment and maintenance of mitochondrial morphology. In yeast, mitochondria form a reticulum, the continuity of which depends on a balance in the frequency of mitochondrial fusion and fission. These events are controlled by the two evolutionarily-conserved GTPases, Fzolp and Dnmlp, and by additional interacting components (Nunnari et al. (1997) Mol. Biol. Cell 8, 1233-1242; for review, see Shaw and Nunnari (2002) TICN 12, 178-184). A systematic screen of a yeast mutant library of deleted nonessential genes has uncovered potentially novel components required for maintenance of mitochondrial morphology and is described in "Genetic basis of mitochondrial function and morphology in Saccharomyces cerevisiae" (Dimmer et al. (2002) Mol. Biol. Cell 13, 847-853). Mitochondria in mutants from this screen are depicted on the cover.Jodi Nunnari