SYTL1 (synaptotagmin-like 1) is a RAB27A effector protein that regulates vesicle trafficking and exocytosis. Structurally, SYTL1 binds phosphatidylinositol 3,4,5-trisphosphate and functions downstream of the small GTPase RAB27A to facilitate transport of cytoplasmic vesicles to the plasma membrane 1. In immune cells, SYTL1 mediates cytotoxic granule exocytosis in lymphocytes 2. Mechanistically, SYTL1 operates within the RAB27/SYTL axis to control intracellular trafficking of membrane proteins and secretory cargo. The protein participates in secretory autophagy in retinal pigment epithelium as part of an AKT2-SYTL1-TRIM16-SNAP23 complex 3, and regulates CXCR4 trafficking to modulate cell migration 4. SYTL1 dysregulation associates with multiple diseases. In acute myeloid leukemia, MEIS1-induced SYTL1 promotes leukemic cell homing and engraftment via CXCL12/CXCR4 signaling 4, while RAB27A/SYTL1 upregulation enhances tumor microenvironment remodeling 5. In influenza virus infection, SYTL1 facilitates viral membrane protein transport and assembly 1. Conversely, reduced SYTL1 expression in bladder cancer impairs natural killer cell anti-tumor responses 6, and SYTL1 downregulation correlates with poor prognosis in endometrial cancer 2. Clinically, SYTL1 represents a prognostic biomarker in multiple cancers and a potential therapeutic target for regulating vesicle trafficking in viral infections and malignancies.