The best-characterized MAPK modules are the ERK pathway, the SAPK/JNK pathway, and the p38 pathway.

The classical Ras-to-MAPK cascade is shown in bold. The MAPK cascades consist of a MAPKKK, a MAPKK, and a MAPK. MAPKKKs are activated through a large variety of extracellular signals such as growth factors, differentiation factors, and stress. The activated MAPKKK can phosphorylate and activate 1 or several MAPKKs, which, in turn, phosphorylate and activate a specific MAPK. Activated MAPK phosphorylates and activates various substrates in the cytoplasma and the nucleus of the cell, including transcription factors. These downstream targets control cellular responses (eg, apoptosis, proliferation, and differentiation). Thick arrows connect the signaling proteins with their preferred substrates (effectors). Note the complexity and the potential for crosstalk between the pathways.