How an external signal becomes a change in a cell's genes

The fate of a cell — which specialized type it will become — depends on which of its genes are turned on or off. Gene activity is the switch that settles the outcome. What throws the switch is a relay. External signals travel through multi-step sequences called signaling pathways, which translate an incoming signal into specific changes in gene activity inside the cell. That translation step is the thing to hold onto: a chemical signal on one side, a different kind of instruction — a change in which genes are active — on the other.