When a cell calls for a specific protein, resting genes go into high gear to start the process. Because a cell's protein-making machinery cannot directly interpret the instructions encrypted in DNA, an enzyme converts the directives into a message known as mRNA. Typically made up of only one strand, mRNA travels from the protein factory elsewhere in the cell through openings in the nuclear membrane. About half of human genes control processes in the brain and other parts of the nervous system. Scientists are developing new methods to analyze the protein-making activity of the full complement of an organism's genes on a silicon chip that is no larger than a postage stamp.