SYTL4 (synaptotagmin-like 4) is a Rab27 effector protein that regulates exocytosis and vesicular transport across multiple biological contexts. In its primary function, SYTL4 modulates the secretion of dense-core granules and hormones in pancreatic and pituitary tissues 1. Mechanistically, SYTL4 functions as a Rab27a effector to facilitate MVE docking at the plasma membrane and promote exosome secretion 1, and operates downstream of Rab27a to regulate transport of influenza virus membrane proteins to the cell surface 2. Additionally, SYTL4 mediates exocytosis of CXCL8 chemokines in pancreatic cancer, supporting tumor progression 3. Disease relevance is substantial. SYTL4 is overexpressed in multiple cancer entities and associated with poor prognosis in triple-negative breast cancer (TNBC), where it confers paclitaxel resistance by directly binding microtubules and inhibiting their polymerization 4. In pancreatic cancer, SYTL4 drives gemcitabine resistance and immune evasion 3. Beyond cancer, SYTL4 variants have been implicated in autism spectrum disorder through disruption of RAB-binding domains and interactions with autism-associated genes 5. SYTL4 peptides show elevated plasma levels in Alzheimer's disease patients 6. Clinically, targeting SYTL4 represents a therapeutic opportunity in chemoresistant cancers and viral infections 2, though further validation is needed for neurodevelopmental applications.