Understanding how chromosomes separate during cell division could help scientists understand birth defects such as Down's syndrome, where chromosomes do not separate properly, and how to stop cancer cells dividing. When cells divide, the pairs of chromosomes that contain their DNA are pulled apart into two new cells. Researchers at the University of California, Davis, have now shown that a protein called dynein is the motor that drags the chromosomes apart. "A major problem in cell biology is how replicated chromosomes move to the spindle poles," says cell biologist Jonathan Scholey. "People have looked for this role for dynein for years, so it's a very important result." Early in the process of cell division, microtubule filaments fan out from each end of the cell to form a structure called the spindle. These filaments capture the chromosomes and somehow move them to opposite corners of the cell. Scholey's laboratory studies cell division in fruit-fly embryos, where all the cells divide at the same time. When they injected a chemical that specifically inactivated dynein, they found that close to the injection site, cell division was completely stopped. Further away, there was enough dynein activity to build the spindle, but the chromosomes did not move apart. Scholey's team concluded that dynein was necessary for the chromosomes to move up the filaments. They think that dynein attaches to the chromosome and then "walks" up the microtubule lattice, dragging the chromosome with it. The research will be published in the December issue of Nature Cell Biology. The paper is available now online at .