The Ras protein is a member of a large family of small GTP-binding proteins, often called monomeric GTPases to distinguish them from the trimeric G proteins. Ras resembles the α subunit of a G protein and functions as a molecular switch in much the same way.

It cycles between two distinct conformational states—active when GTP is bound and inactive when GDP is bound. Interaction with an activating protein called Ras-GEF encourages Ras to exchange its GDP for GTP, thus switching Ras to its activated state; after a delay, Ras is switched off by a GAP called Ras-GAP, which promotes the hydrolysis of its bound GTP to GDP.

In its active state, Ras initiates a phosphorylation cascade in which a series of serine/threonine kinases phosphorylate and activate one another in sequence, like an intracellular game of dominoes. This relay system, which carries the signal from the plasma membrane to the nucleus, includes a three-kinase module called the MAP-kinase signaling module, in honor of the final enzyme in the chain, the mitogen-activated protein kinase, or MAP kinase.

In this pathway, MAP kinase is phosphorylated and activated by an enzyme called, logically enough, MAP kinase kinase. This protein is itself switched on by a MAP kinase kinase kinase (which is activated by Ras). At the end of the MAP-kinase cascade, MAP kinase phosphorylates various effector proteins, including certain transcription regulators, altering their ability to control gene transcription. The resulting change in the pattern of gene expression may stimulate cell proliferation, promote cell survival, or induce cell differentiation: the precise outcome will depend on which other genes are active in the cell and what other signals the cell receives.