The diagnosis of malignant mesothelioma may be favored by the detection of extensive tumor necrosis, marked cytological atypia, absence of marked nuclear pleomorphism, absence of a high mitotic rate, focal biphasic growth pattern, infiltrative growth pattern, prominent tubulopapillary pattern, polygonal cells with eosinophilic cytoplasm and the presence of intracellular acid (PAS negative) mucin rather than neutral (PAS positive) mucin. This evidence concerns the gene MUC5AC and malignant mesothelioma.