Mitogen-activated protein kinase 8 (also known as JNK1) is a ubiquitous enzyme that is a member of the MAP kinase and JNK family. MAP kinases act as an integration point for multiple biochemical signals, and are involved in a wide variety of cellular processes such as proliferation, differentiation, transcription regulation and development. This kinase is activated by various cell stimuli, and targets specific transcription factors, and thus mediates immediate-early gene expression in response to cell stimuli. The activation of this kinase by tumor-necrosis factor alpha (TNF-alpha) is found to be required for TNF-alpha-induced apoptosis. This kinase is also involved in UV radiation-induced apoptosis, which is thought to be related to the cytochrome c-mediated cell death pathway. Studies of the mouse counterpart of this gene suggested that this kinase play a key role in T cell proliferation, apoptosis and differentiation. MAPK8 contains multiple amino acid sites that are phosphorylated and ubiquitinated.