The STE group includes many protein kinases involved in MAP kinase cascades. Mitogen-activated protein kinases (MAP kinases, CMGC group) play a central role in the transduction of various extra- and intracellular signals and are evolutionarily conserved from Trypanosomatids to mammals, but have also been lost secondarily in reduced kinomes (e.g. Plasmodium falciparum and the fungi-like intracellular parasite Encephalitozoon cuniculi).

These typically operate in a cascade of networks, where a MAP kinase (MAPK) is phosphorylated and activated by a MAPK kinase (MAPKK), which itself is activated by a MAPKK kinase (MAPKKK). Abnormalities in MAP kinase cascades are tightly linked to oncogenic transformation.

In plants, MAP kinases are represented by multigene families and are organized into a complex network that enables the efficient transmission of specific stimuli. In plants, it is known that signaling for cell division and stress responses is mediated through MAP kinases, and even auxin, ABA and possibly ethylene and cytokinin also utilize a MAP kinase pathway. Most of the biotic (pathogens and pathogen-derived elicitors) including wounding and abiotic stresses (salinity, cold, drought, and oxidative) can induce defense responses in plants through MAP kinase pathways.