STT3A is the catalytic subunit of the oligosaccharyltransferase complex (OST-A) that catalyzes the initial transfer of a defined glycan (Glc₃Man₉GlcNAc₂) from dolichol-pyrophosphate to asparagine residues within Asn-X-Ser/Thr motifs in nascent polypeptide chains during cotranslational protein N-glycosylation 1. STT3A contains the active site and substrate-binding pockets and associates with the Sec61 translocon complex at the endoplasmic reticulum membrane 2. While STT3A-containing OST-A complexes mediate cotranslational N-glycosylation of most sites, STT3A also prevents hyperglycosylation by restricting glycosylation of facultative sites before protein folding is complete, working alongside chaperones like HSP90B1 1. Beyond housekeeping functions, STT3A activity is regulated by specificity factors controlling cell surface receptor signaling and tissue development 1. Notably, STT3A mediates N-glycosylation of PD-L1, a key immune checkpoint ligand, through recruitment by signaling molecules, thereby promoting tumor immune evasion in multiple cancer contexts 3456. The catalytic site represents a druggable target, with inhibitors like NGI-1 showing promise for therapeutic development 7. Mutations in STT3A cause congenital disorders of glycosylation type 1W, demonstrating its essential role in human health 8.