OST4 is a small, non-catalytic subunit of the oligosaccharyltransferase (OST) complex that plays a critical role in N-linked protein glycosylation in the endoplasmic reticulum 1. The OST complex catalyzes the transfer of oligosaccharides from dolichol-pyrophosphate to asparagine residues in nascent polypeptides, representing the initial step in protein N-glycosylation 2. OST4 specifically functions to stabilize interactions between OST subunits, particularly bridging the catalytic STT3 subunit with other components like ribophorin I 1. The protein adopts an α-helical transmembrane structure with a functionally important kink that may be crucial for its stabilizing role 3. OST4 associates with both STT3A and STT3B isoforms and is essential for maintaining OST complex stability, as its depletion leads to complex destabilization and impaired N-glycosylation of endogenous proteins like prosaposin 1. The gene shows consistent high expression in healthy platelets and serves as a reliable reference gene in extracellular vesicles 45. Mutations in the transmembrane region, such as V23D, can severely impair OST function and cause temperature-sensitive phenotypes 67, highlighting its critical role in cellular glycosylation processes.