Some signaling molecules are able to cross the plasma membrane and bind to intracellular receptors in the cytoplasm or nucleus, whereas most bind to receptors expressed on the target cell surface. The sections that follow discuss the major types of signaling molecules and the receptors with which they interact.

Ligand-activated receptors convey signals into the cell by activating signaling pathways that ultimately affect cytosolic machineries or nuclear transcriptional programs or by directly translocating to the nucleus to regulate transcription.

In most cases, a chain of reactions transmits signals from the cell surface to a variety of intracellular targets—a process called intracellular signal transduction. The targets of such signaling pathways frequently include transcription factors that function to regulate gene expression.

When a water-soluble ligand binds to a cell-surface receptor, it activates the receptor, initiating a signal transduction pathway that alters cellular function. The 3 types of cell-surface receptors include G protein–coupled receptors (GPCRs), ion channel receptors, and enzyme-linked receptors.