Several mammalian kinases have been identified which exhibit sequence similarity to the Saccharomyces cerevisiae serine/threonine kinase STE20. STE20 is involved in relaying signals from G-protein coupled receptors to cytosolic MAP kinase cascades, and it lies upstream of a MAP kinase kinase kinase. Mammalian STE20-like kinases include NIK, KHS, GLK, YSK1, HPK1, Krs-1, Krs-2, and GC kinase. NIK (Nck interacting kinase), like many of the STE20-like kinases, has been shown to activate the SAPK/JNK stress response pathway. Both the kinase domain and the C-terminal regulatory domain of NIK are required for full activation. NIK interacts with MEKK1 and is thought to act upstream of MEKK1 in the SAPK/JNK signaling pathway.