The JNK group of MAP kinases plays a critical role in signal transduction in response to various cellular stimuli. This review summarizes recent advances in understanding the JNK signaling pathway, highlighting its involvement in cell proliferation, apoptosis, and differentiation. JNK is activated by dual phosphorylation on Thr and Tyr, mediated by a kinase cascade involving MAPKKK, MAPKK, and MAPK. JNK is essential for embryonic morphogenesis and contributes to the regulation of cell proliferation and apoptosis. It also plays a role in the function of some differentiated cells. JNK regulates AP-1 transcription activity by phosphorylating c-Jun and other AP-1 proteins, which enhances their transcriptional activity. This regulation is supported by genetic analyses in Drosophila and murine cells. JNK is required for AP-1 activation caused by stress and some cytokines, but not for other stimuli. The JNK signaling pathway is implicated in multiple physiological processes, including cell survival and tumor development. The JNK protein kinases are encoded by three genes, with Jnk1 and Jnk2 expressed ubiquitously and Jnk3 restricted to specific tissues. These genes produce multiple isoforms through alternative splicing, which influences substrate specificity. JNK is activated by cytokines and environmental stress, and its activity is regulated by various phosphatases. JNK is essential for the differentiation of CD4 T helper cells into effector cells, but not for their activation. JNK is activated by two dual-specificity protein kinases, MKK4 and MKK7, which are regulated by MAPKKK. These kinases are involved in various physiological processes, including embryonic development and immune responses. Targeted gene disruption studies in mice have shown that MKK4 and MKK7 are required for embryonic development, with MKK4 deficiency leading to liver apoptosis. The JNK pathway is activated by a large group of MAPKKK, including MEKK, MLK, ASK, TAK1, and TPL2. These MAPKKK are involved in various signaling pathways, including those mediated by Rho family GTPases and TRAF adaptor proteins. The activation of JNK by these MAPKKK is crucial for stress-induced apoptosis and other cellular responses. JNK signaling modules are organized by molecular scaffold proteins, such as JIP, which facilitate the assembly of signaling complexes. These scaffolds are essential for the efficient activation of JNK within specific cellular regions. JNK is involved in both apoptosis and survival signaling, with its role depending on the cellular context. JNK is also implicated in tumor development, as it contributes to the survival and proliferation of cancer cells. JNK activity is regulated by various signaling pathways, including NF-κB, Akt/PKB, and ERK. The suppression of JNK-dependent apoptosis is likely to be a key factor in tumor progression. However, the exact mechanisms by which JNK contributes to tumor development are still underThe JNK group of MAP kinases plays a critical role in signal transduction in response to various cellular stimuli. This review summarizes recent advances in understanding the JNK signaling pathway, highlighting its involvement in cell proliferation, apoptosis, and differentiation. JNK is activated by dual phosphorylation on Thr and Tyr, mediated by a kinase cascade involving MAPKKK, MAPKK, and MAPK. JNK is essential for embryonic morphogenesis and contributes to the regulation of cell proliferation and apoptosis. It also plays a role in the function of some differentiated cells. JNK regulates AP-1 transcription activity by phosphorylating c-Jun and other AP-1 proteins, which enhances their transcriptional activity. This regulation is supported by genetic analyses in Drosophila and murine cells. JNK is required for AP-1 activation caused by stress and some cytokines, but not for other stimuli. The JNK signaling pathway is implicated in multiple physiological processes, including cell survival and tumor development. The JNK protein kinases are encoded by three genes, with Jnk1 and Jnk2 expressed ubiquitously and Jnk3 restricted to specific tissues. These genes produce multiple isoforms through alternative splicing, which influences substrate specificity. JNK is activated by cytokines and environmental stress, and its activity is regulated by various phosphatases. JNK is essential for the differentiation of CD4 T helper cells into effector cells, but not for their activation. JNK is activated by two dual-specificity protein kinases, MKK4 and MKK7, which are regulated by MAPKKK. These kinases are involved in various physiological processes, including embryonic development and immune responses. Targeted gene disruption studies in mice have shown that MKK4 and MKK7 are required for embryonic development, with MKK4 deficiency leading to liver apoptosis. The JNK pathway is activated by a large group of MAPKKK, including MEKK, MLK, ASK, TAK1, and TPL2. These MAPKKK are involved in various signaling pathways, including those mediated by Rho family GTPases and TRAF adaptor proteins. The activation of JNK by these MAPKKK is crucial for stress-induced apoptosis and other cellular responses. JNK signaling modules are organized by molecular scaffold proteins, such as JIP, which facilitate the assembly of signaling complexes. These scaffolds are essential for the efficient activation of JNK within specific cellular regions. JNK is involved in both apoptosis and survival signaling, with its role depending on the cellular context. JNK is also implicated in tumor development, as it contributes to the survival and proliferation of cancer cells. JNK activity is regulated by various signaling pathways, including NF-κB, Akt/PKB, and ERK. The suppression of JNK-dependent apoptosis is likely to be a key factor in tumor progression. However, the exact mechanisms by which JNK contributes to tumor development are still under