Many of us will be migrating somewhere this month — to the American Society for Cell Biology Annual Meeting in San Francisco, perhaps, or to celebrate the holiday season.

Cells can also migrate — indeed, cell migration is essential for life, as it is an integral feature of development, repair and defence processes. Migration of whole cells requires cell polarization, a process that is mediated by microtubules. On page 957, J. Victor Small and colleagues propose a mechanism for how microtubules exert this role by modulating the pattern of adhesions that a cell forms with its underlying matrix, through tractional forces.

Molecules can migrate across cell borders, an example of which Rojas and Apodaca describe on page 944. Immunoglobulins are transported across epithelial cells through a process called transcytosis, and secreted into the mucosal lumen, where they carry out important immune functions. Cell invasion by digestion of the extracellular matrix is an altogether different migration event. The urokinase receptor uPAR was originally thought to assist in extracellular protein digestion, but — as Blasi and Carmeliet report on page 932 — uPAR has many more roles, for example in various signalling pathways.

Migration of molecules within cells often involves membrane trafficking. On page 919, Worby and Dixon describe the family of sorting nexins, which has been implicated in membrane trafficking and protein sorting. On page 971, Beverly Wendland discusses whether epsins might be part of a growing family of adaptor proteins that select specific cargo for endocytosis. Finally, on page 893, Scott Emr and colleagues review recent advances in the understanding of protein sorting into multivesicular bodies, which is important for the downregulation of activated signalling receptors and stimulation of the immune response.