Mesothelin is highly expressed in epithelial malignant mesothelioma. In the original study by Chang and colleagues mesothelin expression was evaluated by mAb-K1 using frozen section tissues of patients with malignant mesothelioma.12 Out of the 23 pleural mesothelioma samples analysed all 15 epithelial mesothelioma had mesothelin expression, while 4 sarcomatous mesotheliomas were negative. In the four samples with biphasic mesothelioma only the epithelial component stained for mesothelin. Mesothelin expression in paraffin embedded mesothelioma tissue samples was studied by Ordonez using mAb 5B2.3 Out of the 55 mesothelioma specimens (44 epithelioid, 3 biphasic, and 8 sarcomatoid) studied mesothelin reactivity was noted in all epithelioid mesotheliomas and the epithelial component of biphasic mesotheliomas. However, none of the sarcomatous mesotheliomas expressed mesothelin. These results are in agreement with the results of Chang and colleagues that mesothelin is present in all epithelial mesotheliomas and is absent in the sarcomatous type. Although mesothelin is not a specific marker for mesothelioma a negative mesothelin immunostain strongly argues against the diagnosis of epithelioid mesothelioma.13