Reports: “Mitotic Golgi partitioning is driven by the membrane-fissioning protein CtBP3/BARS” by C. Hidalgo Carcedo et al. (2 July, p. 93). This paper reported that BARS is crucially involved in mitotic Golgi partitioning and entry into mitosis. CtBP3/BARS (BARS) is a protein involved in Golgi membrane fission [S. Spanò et al., J. Biol. Chem.274, 17705 (1999); R. Weigert et al., Nature402, 429 (1999)] and a member of the CtBP family that comprises CtBP1 and CtBP2, both of which are transcriptional co-repressors [G. Chinnadurai, Mol. Cell9, 213 (2002); G. Chinnadurai, Bioessays. 25, 9 (2003)]. BARS is almost certainly a ctbp1 gene product and therefore a splice variant of CtBP1. A KO mouse has been generated in which both ctbp1 and ctbp2 have been deleted (and which therefore also lacks BARS). This KO is embryonically lethal, but cells derived from these embryos proliferate normally, indicating that the partitioning of their Golgi complex should occur during mitosis (although it is not clear that the Golgi partitions normally) [J. D. Hildebrand, P. Soriano, Mol. Cell. Biol.22, 5296 (2002)]. This is apparently discrepant with the report by Hidalgo Carcedo et al. that BARS is crucially involved in mitotic Golgi partitioning and entry into mitosis. Similar discrepancies between KO and classical cell biological studies in cultured cells are frequent and provide useful insights into the process under study [M. Pagano, P. K. Jackson, Cell118, 535 (2004)]. The following is a brief discussion of a few hypotheses that can cast light on this specific case. First, more than one fission mechanism might be involved in mitotic Golgi partitioning. For instance, mitotic Golgi fragmentation involves two stages, one consisting of the consumption of Golgi membranes by the irreversible budding of COPI vesicles, and the second, of the tubulation of Golgi cisternae followed by their cleavage into smaller pieces [J. Shorter, G. Warren, Annu. Rev. Cell. Dev. Biol.18, 379 (2002)]. The latter component is likely to be the one that is dependent on BARS. It is possible that in embryonic cells lacking BARS, the COPI-dependent mechanism might carry the Golgi partitioning process far enough to allow mitosis to proceed. Another possibility is that in these cells, once the Golgi cisternae have been transformed into tubules during mitosis (presumably via phosphorylation of the relevant golgins) (Shorter and Warren), a dynamin-like protein is able to cleave these tubes into small pieces. Second, BARS might not be a core fission protein, but rather a regulator, which could be replaced in BARS-null cells by a related gene with a similar function. Finally, it is possible that the Golgi structure in embryonic cells is organized differently and does not require the BARS-controlled machinery to enter mitosis. This last possibility is supported by morphological studies that are presently in progress. The above mechanisms (and possibly others) might allow the cells to undergo mitosis and execute Golgi partitioning even in the absence of BARS. This could result in mitotic Golgi phenotypes that might or might not be different from those in control cells.