In our data, the particularly high positivity rate of Upk3b in epithelioid mesotheliomas (82.1%) in combination with the complete absence of Upk3b in pulmonary adenocarcinomas (0/184) and in other adenocarcinomas that are known to frequently metastasize to the lung, such as prostate cancer (0/491), breast cancer (0/706), as well as colorectal (0/2483), gastric (0/474), and pancreatic adenocarcinomas (0/734) permits the use of Upk3b as a putative diagnostic marker for the difficult distinction of malignant mesothelioma from primary or metastatic adenocarcinomas in the lung. The gene discussed is UPK3B; the disease is pancreatic adenocarcinoma.