S100P is a member of the S100 family of proteins that contains 2 EF-hand calcium-binding motifs and that is involved in the regulation of a number of cellular processes such as cell cycle progression and differentiation. It is expressed in the cytoplasm and/or nuclei of a variety of cells. S100P was initially identified in the placenta at rather high levels, in pancreatic ductal adenocarcinoma in pancreatic resection, and in fine needle aspiration biopsy specimens, and in the invasive component of cells of intraductal papillary mucinous neoplasms, including perineural, lymphatic, and minimal invasion. This antibody can be helpful in distinguishing adenocarcinoma from reactive epithelial changes on challenging bile duct biopsies, as well as urothelial carcinomas from other genitourinary neoplasms that enter into the differential diagnosis.