CLDN1 and adenoma: In 2005, Tsukahara and co-workers reported claudin-1 expression in hyperplastic lesions in the pancreas in 50% (3 out of 6) hyperplastic foci, 82% (14 out of 17) adenomas, 30% (3 out of 10) borderline tumors, 2 out of 6 (33%) non-invasive carcinomas, and 1 out of 5 invasive pancreatic cancers (20%), producing a statistically negative association with progression of malignancy.