AG-2 (Anterior gradient protein 2; also AGR2, GOB4 and HPC8) is an 18-21kD member of the PDI family of enzymes. It is expressed in secretory cell types, such as prostate epithelium, small intestine goblet, Paneth and enteroendocrine cells, and multiple carcinoma cell types. Although AG-2 possesses a signal sequence, it appears to be rarely secreted, instead being found in the ER due the presence of a KTEL ER retention signal at its C-terminus. AG-2 forms transient disulfide linkages with molecules destined for secretion, possibly aiding protein folding. Mature human AG-2 is 155aa in length (aa21-175). Cys81 is presumed to participate in intermolecular bond formation. There is one potential alternative start site 20aa upstream of the standard start site.