Ras gene mutations in intraductal papillary neoplasms of the pancreas. Analysis in five cases. Five intraductal papillary neoplasms of the pancreas were analyzed for the presence of the Ras gene mutations. Three (60%) of the five neoplasms showed point mutations at K-ras codon 12. This incidence of the mutations was rather low compared with that found with ordinary pancreatic adenocarcinoma. The presence of the mutations did not correlate with the severity of cellular atypia, but was apparently related to the size of the tumor. The two neoplasms that had no mutations were smaller than the others that had mutations. The analysis suggested that Ras gene mutation is not the first genetic alteration of tumor progression, but that it occurs during the development of neoplasms of the pancreas. 