As we have seen in the discussion of stem cells, differentiated cells can be reprogrammed by the action of only a few key genes or small organic molecules. In some cases, the processes that push differentiation in a forward direction are also quite simple. An important example is signal transduction, in which an extracellular molecule acts as a signal to activate a membrane protein that in turn activates molecules inside the cell that control differentiation (Chapter 9). The signaling molecule is called the ligand and the membrane protein that it activates is called the receptor. The following example shows how a simple ligand–receptor pair can have profound effects not only on the differentiation of the cell that carries the receptor, but also on its neighbors.