During mitosis, the entire content of the cell must be divided equally between two daughter cells before karyoand cytokinesis are completed. How is this complex process accomplished? In the case of the Golgi, it was found many years ago in tissue culture cells that the characteristic stack-like organization of the Golgi fragmented at the onset of mitosis (Robbins and Gonatas, 1964; Lucocq and Warren, 1987), and that Golgi fragments were subsequently distributed to each daughter cell. The identity of these Golgi fragments and the mechanisms involved in Golgi fragmentation and in the segregation of membranes and Golgi contents to daughter cells are currently the subject of considerable study and debate. In this issue of JCB, three papers are published on Golgi fragmentation during mitosis (Kano et al., 2000; Lowe et al., 2000; Colanzi et al., 2000), which, together with papers published recently in Cell (Zaal et al., 1999) and Molecular Biology of the Cell (Terasaki, 2000), provide new insights into Golgi fragmentation, the mechanisms involved, and the experimental difficulties in analyzing this process. The background of these papers is also discussed, however, this commentary is not an exhaustive review of the literature on Golgi inheritance during mitosis (see Warren and Wickner, 1996). Nor is it a treatise on different models of Golgi inheritance or the pros and cons for those models. It is written from the perspective of an outsider, like most of you reading this, who wants to understand from the printed word and published data what happens to the Golgi during mitosis.

We will consider two points. First, changes in Golgi organization during mitosis: we will discuss evidence for different stages in Golgi fragmentation, whether there is retrograde membrane flow from the Golgi to the ER, and the possible significance of the ER as an end-point in this process. Second, mechanisms underlying these changes: we will discuss the evidence for roles of MEK1 (mitogen-activated protein kinase kinase, MAPKK) 1 and Cdc2, whether these kinases work alone or sequentially in Golgi fragmen-