P4HB and breast cancer: Belonging to the protein disulfide isomerase (PD 1) family, the anterior gradient 2 (AGR2) is an estrogen-responsive developmentally regulated gene.117 First recognised in breast cancer cells as an estrogen receptor target,118 but now also observed as overexpressed in other tumours like ovarian,119 pancreatic,120 and colorectal cancers.121