Like tractor-trailers on a highway, small sacs called vesicles transport substances within cells. This image tracks the motion of vesicles in a living cell. The short red and yellow marks offer information on vesicle movement. The lines spanning the image show overall traffic trends. Typically, the sacs flow from the lower right (blue) to the upper left (red) corner of the picture. Such maps help researchers follow different kinds of cellular processes as they unfold. Courtesy of postdoctoral fellow Alexey Sharonov and chemist Robin Hochstrasser, both at the University of Pennsylvania, who also collaborate in a cellular imaging project supported by the NIH Roadmap for Medical Research.