FYN is a non-receptor tyrosine kinase of the Src family that phosphorylates diverse cellular substrates to regulate multiple biological processes. FYN functions in cell adhesion and motility by phosphorylating adhesion molecules including β-catenin and δ-catenin 1, and regulates cytoskeletal remodeling through phosphorylation of actin regulators and microtubule-associated proteins 23. In immune signaling, FYN participates in T-cell receptor (TCR) signaling by phosphorylating downstream effectors including DAB1 in reelin signaling and regulating both positive and negative feedback loops through phosphorylation of PAG1 and PDCD1 45. FYN also modulates natural killer cell activation through CD244 signaling 6. Beyond immune function, FYN regulates glomerular slit diaphragm integrity by phosphorylating components including NPHS1 and TRPC6 78, and promotes neuronal processes by phosphorylating neural proteins such as DPYSL2 and SNCA 910. Dysregulation of FYN is implicated in multiple pathologies: elevated FYN expression promotes proliferation and metastasis in gastric cancer through TOPK phosphorylation 11, FYN polymorphisms associate with increased thyroid cancer risk 12, and FYN acts as a risk factor in Parkinson's disease through microglial Kv1.3 channel modulation 13. Additionally, elevated FYN transcription drives autoimmunity and Th2 polarization in IgG4-related disease 14, highlighting FYN's importance as both a therapeutic target and prognostic marker across diverse diseases.