JAK

Janus kinases (JAKs) are receptor-associated tyrosine kinases that mediate signal transduction for type I and II cytokine receptors, regulating immune responses and hematopoiesis^[1][2]. JAKs phosphorylate receptor cytoplasmic domains, activating downstream STAT transcription factors that control gene expression involved in inflammation, cell proliferation, and differentiation^[1][3][4]. Among the four JAK isoforms (JAK1, JAK2, JAK3, TYK2), each exhibits distinct cytokine specificity and receptor pairing, influencing pathway selectivity and disease outcomes^[1][5][6]. Dysregulated JAK-STAT signaling contributes to autoimmune disorders, hematologic malignancies, and inflammatory conditions such as rheumatoid arthritis, inflammatory bowel disease, and spondyloarthritis^[2][3][7][8]. Mechanistically, TYK2 and JAK1 JH2 pseudokinase domains act as allosteric regulators, offering targets for isoform-selective inhibition^[5][6]. Small-molecule JAK inhibitors, including tofacitinib, upadacitinib, and filgotinib, block cytokine-driven signaling by interfering with kinase activity or pseudokinase domain function, providing oral therapeutic options with defined isoform selectivity^{[9][10][11][12]}. Experimental applications exploit JAK inhibitors to modulate B cell and T cell functions, study inflammatory pathways, and explore CNS and skeletal muscle pathophysiology, highlighting their utility in both preclinical and clinical research contexts^[3][13][4]. Compared with broad inhibitors, next-generation allosteric and pseudokinase-targeting molecules reduce off-target effects while maintaining pathway-specific efficacy^[12][6]. Collectively, JAK-targeted modulation represents a mechanistic strategy for precise intervention in cytokine-mediated diseases.