Protein OS-9 is a protein that in humans is encoded by the OS9 gene.This gene encodes a protein that is highly expressed in osteosarcomas. This protein binds to the hypoxia-inducible factor 1 (HIF-1), a key regulator of the hypoxic response and angiogenesis, and promotes the degradation of one of its subunits. Alternate transcriptional splice variants, encoding different isoforms, have been characterized.