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Cover  Molecular motors participate in a variety of cellular processes by generating force against cytoskeletal tracks. Tracks, such as microtubules, have an inherent polarity, a plus and a minus end. Understanding the physiological role of motors has required determining the direction they generate force along these tracks. The accompanying micrographs illustrate how the direction of force generation was determined for flagellar dyneins. Using poly-L-lysine-coated microscope grids, Sale and Satir (1977, Proc. Natl. Acad. Sci. USA 74, 2045-2949) were able to capture and observe the results of dynein-driven microtubule sliding using electron microscopy. The polarity of the flagellar microtubules could then be determined by several structural criteria. Illustrated are two images from that study demonstrating that dynein and its microtubule cargo (labeled R in each image) were translocated toward the minus end of the adjacent microtubule track (labeled L). These results provided the first demonstration that dynein is a minus end-directed motor. The same polarity of force generation has since been confirmed for dyneins from a number of sources (Fox and Sale [1987], J. Cell Biol. 105, 1781-1787; Paschal and Vallee [1987], Nature 330, 181-183; Schnapp and Reese [1989], Proc. Natl. Acad. Sci. USA 86, 1548-1552). Reproduced from Proceedings of the National Academy of Sciences with permission of the authors.Elizabeth Smith