OS9 is an ER-resident lectin that serves as a critical component of the HRD1 ubiquitin ligase complex, which mediates ER-associated degradation (ERAD) of misfolded glycoproteins 1. OS9 recognizes and binds improperly folded and hyperglycosylated proteins through its lectin domains, retaining them in the ER lumen and facilitating their transfer to the ubiquitination machinery for proteasomal degradation 1. The structural basis for its function involves a claw-like configuration with SEL1L in the ER lumen that mediates substrate engagement 1. OS9 redundantly promotes glycoprotein degradation with the related lectin XTP3B and acts antagonistically to XTP3B in decoding non-glycosylated protein degrons 2. When ERAD function is impaired through OS9 depletion, canonical ERAD substrates are alternatively delivered to degradative endolysosomes through ER-to-lysosome-associated degradation pathways as a compensatory mechanism 3. Beyond quality control, OS9 has clinical relevance in viral infections, with pathogens such as Japanese encephalitis virus hijacking OS9 and associated ERAD machinery to promote viral replication 4. Disease-associated mutations affecting OS9 stability or complex assembly contribute to human pathology 1.