MEK (Mitogen-Activated Protein Kinase Kinase), primarily consisting of MEK1 and MEK2, is a class of dual-specificity kinases that specifically phosphorylate serine and threonine residues of ERK. MEK localizes inside the cell, predominantly functioning in the cytoplasm, where it participates in the MAPK signaling pathway by activating ERK, thereby regulating cell proliferation, differentiation, and survival. MEK is expressed in various cell types and tissues, with its activation and inhibition tightly controlled by upstream Ras-Raf signaling. Gene-specific expression and functional mutations of MEK are associated with various diseases, including cancer. In certain cancers, abnormal activation of the MEK kinase pathway is a critical factor in tumor growth and survival, making it a hotspot for targeted therapy.