MAPK9 (mitogen-activated protein kinase 9) is a member of the JNK (c-Jun N-terminal kinase) signaling pathway that functions as a protein serine/threonine kinase involved in cellular stress responses and apoptosis regulation 1. The protein plays a critical role in apoptotic signaling, where its activation leads to downstream apoptosis pathway activation in hepatocellular carcinoma cells 1. MAPK9 exhibits complex roles in cancer pathogenesis that appear context-dependent: in glioma, elevated MAPK9 expression correlates with poor prognosis and promotes tumor progression through the Wnt/β-catenin-regulated EMT pathway 2, while in colorectal cancer, MAPK9 expression tends to be higher in lower-grade tumors, suggesting a potential protective role in early disease stages 3. The gene is also involved in metabolic regulation, where MAPK9-mediated pathways control de novo lipogenesis in nonalcoholic fatty liver disease 4. Additionally, MAPK9 participates in inflammatory responses, with enhanced expression observed in idiopathic multicentric Castleman disease subtypes 5. Circular RNA derived from MAPK9 (circ_MAPK9) functions as an oncogenic factor in hepatocellular carcinoma by acting as a competing endogenous RNA 6. These findings highlight MAPK9's multifaceted roles in cellular signaling, with therapeutic targeting showing promise in certain cancer contexts.