Claudin-4 was consistently negative in normal and reactive mesothelium as well as in all 82 mesotheliomas but strong reactivity (using anti-claudin-4 primary antibody) was found in the significant majority of serosal metastasis from primary carcinomas particularly lung, breast, gastrointestinal tract, pancreas, ovary and primary peritoneal carcinoma. The gene discussed is CLDN4; the disease is carcinoma.