MYL9 (myosin light chain 9) is a regulatory subunit of myosin II that functions as a key controller of contractile activity in both muscle and non-muscle cells 1. Its primary role involves regulation of actomyosin cytoskeleton dynamics through phosphorylation-dependent mechanisms, participating in cytokinesis and cell locomotion 2. Mechanistically, MYL9 regulates contractile force generation and cellular tension. In cancer-associated fibroblasts (CAFs), YAP transcription factor controls MYL9 protein levels to modulate matrix stiffening and promote cancer cell invasion 1. Within CAFs, MYL9 interacts with IQGAP1 to regulate secretion of immunomodulatory factors CCL2 and TGF-β1 through the ERK1/2 pathway 3. Additionally, MYL9 participates in a CD69-mediated immune cell migration system, where platelet-derived MYL9 forms net-like structures facilitating T cell infiltration into inflamed tissues 2. Clinically, elevated MYL9 expression correlates with poor prognosis across multiple cancers and associates with an immunosuppressive microenvironment characterized by M2 macrophage infiltration 3. MYL9+ CAF populations promote drug resistance in EGFR-mutant lung cancer through mitochondrial transfer to persister cells 4. MYL9 is also implicated in vascular smooth muscle cell phenotype switching; Anxa1 deficiency-induced downregulation of MYL9 exacerbates acute aortic dissection progression 5. Dysregulation via the DDX3X-TLE2-MYL9 axis enhances pancreatic cancer metastasis through F-actin remodeling 6. Clinically, MYL9 serves as a pan-cancer prognostic biomarker associated with altered immune infiltration patterns 7.