Ductal pancreatic carcinoma (DPC) is a deadly disease with an incidence of 9 cases in 100,000 people per year and a mortality rate close to 100%. Allelic losses in the long arm of chromosome 9 are commonly encountered in many human malignancies but no data are yet available about DPC. We screened 40 laser-microdissected DPC samples and 6 pre-invasive lesions for 9 microsatellite mapping markers of region 9q21.3 through 9q34.2. A small overlapping region of deletion, spanning 8 million base pairs, was identified between D9S127 and D9S105. Two genes, RSG3 and KLF4, mapped to 9q31.1 through 9q32, were further investigated. A highly significant association was found between KLF4 gene expression levels and genomic status. Similarly, absence of immunohistochemical expression of KLF4 protein was found in 86.8% cases of DPC (33/ 38). Overexpression of KLF4 in a human pancreatic carcinoma cell line induced a significant decrease in the proliferation associated with up-regulation of p21 and the down-regulation of cyclin D1. In conclusion, we identified a novel oncosuppressor region located at the 9q 31.1-3 locus that is lost in DPC at high frequency. Loss of KLF4 expression is closely related to the genomic loss, and its restoration inhibits cancer cell proliferation, suggesting a key suppressor role in pancreatic tumorigenesis. Copyright © 2011 American Society for Investigative Pathology. Published by Elsevier Inc. All rights reserved.