Subclassification of intraductal papillary mucinous neoplasms of the pancreas (IPMNs), based on morphological features and immunohistochemical profiles, has been proposed. Intestinal-type IPMNs frequently show moderate to severe dysplasia. Regenerating islet-derived family, member 4 (REG4) is associated with the adenoma-carcinoma sequence in colon cancer and it is also associated with intestinal phenotype. Therefore, to identify REG4 expression in IPMNs may be helpful to detect high-grade IPMNs. We also investigated REG4 expression and CDX2 expression in IPMNs. To investigate the expressions of REG4 and CDX2 in IPMNs and in invasive ductal adenocarcinoma derived from IPMN, we used immunohistochemical staining and microdissection-based quantitative real-time reverse transcription-polymerase chain reaction. Among 125 IPMNs, 43 (34%) were positive for REG4 and most of the intestinal-type IPMNs showed its expression (35/38). The positive ratio of REG4 expression in colloid carcinoma (5/7) was significantly higher than that in tubular carcinoma (1/17; P ¼ 0.003). Most of CDX2-positive cases (31/33) expressed REG4 protein, whereas only 12 of 92 CDX2-negative cases did (Po0.001). The levels of REG4 mRNA in intestinal-type IPMN were significantly higher compared to those in gastric-type IPMN or to normal pancreatic ductal epithelium (P ¼ 0.005, P ¼ 0.004, respectively). REG4 expression was observed more frequently in borderline lesions (14/28) and carcinoma (21/45) compared to adenoma (8/52). Using the Ki-67 labeling index, REG4 expression was significantly correlated with proliferative activity in borderline lesions. We conclude that REG4 is involved in the 'intestinal' pathway of carcinogenesis in IPMN. Intraductal papillary mucinous neoplasms (IPMNs) show a wide spectrum of histological differentiation from hyperplasia, adenoma and borderline neoplasm to carcinoma, and the existence of an adenoma-carcinoma sequence is documented. [1][2][3] IPMNs are considered to be precursors of invasive cancer of either tubular ductal adenocarcinoma or colloid (mucinous noncystic) carcinoma. [4][5][6][7] Recently, subclassification of IPMNs, based on morphological features and immunohistochemical profiles, has been proposed. Intestinal-type IPMNs frequently show moderate to severe dysplasia and the intestinal-type associated invasive cancer tends to be colloid carcinoma. [8][9][10][11][12] Meanwhile, most gastric-type IPMNs show low-grade dysplasia and rarely associated with invasive cancer. [8][9][10] IPMNs arising from branch ducts often show gastric-type differentiation and have a less aggressive clinical course. 1,9 Pancreatobiliary-type IPMNs show severe atypia corresponding to in situ carcinoma and is often associated with an invasive tubular type of ductal adenocarcinoma. These findings suggest that each type of IPMN follows a different pathway of carcinogenesis. Adsay et al suggested that the intestinal-type IPMN to colloid carcinoma sequence is a distinct pathway of carcinogenesis involving intestinal-related genes CDX2 an...