The process of switching genes on and off is vital, giving the cell its identity. The set of on/off genes instructs a neuron to be a neuron, and a white cell to be a white cell. During the development of an organism, genes—or rather proteins encoded by genes—tell cells about their relative positions and command their future fates. Genes are turned on and off by external stimuli such as hormones, which also signal changes in a cell’s behavior.
