Involved in intracellular vesicle trafficking and vesicle fusion with membranes. Contributes to the granule exocytosis machinery through interaction with soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins that regulate membrane fusion. Regulates cytotoxic granule exocytosis in natural killer (NK) cells. Interacts with STX1A, STX2 and STX3 (By similarity). Interacts with STX11. Placenta, lung, liver, kidney and pancreas, as well as in peripheral blood lymphocytes. The disease is caused by variants affecting the gene represented in this entry. Belongs to the STXBP/unc-18/SEC1 family.