AGR2 (Anterior gradient protein 2, XAG-2) is a protein disulfide isomerase that is required for MUC2 post-transcriptional synthesis and secretion and may play a role in the production of mucus by intestinal cells. It is also thought to be involved more broadly in cell migration, differentiation and growth, as well as in protein folding. As a biomarker, it is useful for identifying Barrett’s esophagus, where it has up to a 70-fold increase in expression over normal epithelia. AGR2 is also overexpressed in prostate, esophageal, pancreatic and ER-positive breast cancers as well as in cancer cell lines, where it is thought to contribute to invasive potential. In immunohistochemistry of normal tissue, AGR2 is expressed in mucus-secreting tissues including the lungs, colon, stomach, small intestine and prostate, as well as in genitalia. References: Cancer Res. 2008 Jan. 68 (2): 492–7, PMID: 18199544; Prostate. 2011 May;71(6):575-87, PMID: 20945500; Biochem. Biophys. Res. Commun. 1998. 251 (1): 111–6. PMID: 9790916; Gastroenterology. 2006 Sep. 131 (3): 925–33, PMID: 16952561; Cancer Res. 2008 Oct. 68 (19): 7811–8, PMID: 18829536; J. Biol. Chem. 2012. 287 (7): 4773–82, PMID: 22184114.