MSLN and pancreatic adenocarcinoma: [25, 26] By contrast, almost all pancreatic adenocarcinomas express both mesothelin and mutated KRAS. However, the temporal relationship of their expression in tumor development is not suggestive of a regulatory relationship since KRAS mutation is one of the earliest detectable changes in pre-malignant PanIN lesions while mesothelin expression does not occur until much later in adenocarcinoma development.