OS-9 is a ubiquitously expressed ensoplasmic reticulum (ER)-associated protein originally identified as being amplified in certain osteosarcomas. It functions in ER quality control and ER-associated degradation (ERAD). There are three isoforms of this protein: OS-9.1, OS-9.2 and OS-9.3. The longest isoform, OS-9.1, contains 667 aa, OS-9.2 lacks aa 535-589, whereas OS-9.3 lacks aa 456-470 and 535-589. OS-9.1 and OS-9.2 are N-glycosylated, ubiquitously expressed in human tissues, and amplified in tumors. Isoform 2 is the major isoform detected in all cell types examined. This antibody can recognize all three isoforms of OS-9.