A small scratch on the skin can be quite painful. Fortunately, the pain is transitory and dissipates quickly once the wound heals. The healing process is facilitated by neutrophils, a type of white blood cell that removes bacteria and other foreign materials from a wound. Neutrophils normally reside in your circulatory system but, when needed, are able to leave the bloodstream and efficiently navigate through connective tissue to the injured area. How do they figure out where to go? The answer is chemotaxis, the process of cells following chemical gradients. In addition to wound healing, chemotaxis is important to many other biological processes. Chemical information can help sperm find the egg cell during fertilization. In embryonic development, cells are often directed to their proper location through gradients. Chemotaxis can also aid the spread of cancer during metastasis, the process by which cells leave the primary tumor and seed new tumors in other parts of the body. Experiments have shown that gradients of growth factors guide an initial step in the metastatic process; that step involves the movement of malignant cells away from the tumor and toward blood vessels.