HEG1 and sarcomatoid mesothelioma: In addition, the mesothelial population from a significant number of biopsies of fibrous pleuritis showed weak and diffuse HEG1 staining just like a significant number of cases with reactive mesothelial cells showed strong and diffuse HEG1 apical staining; thus, HEG1 showed a low sensitivity in distinguishing sarcomatoid mesothelioma from fibrous pleuritis and a low specificity in distinguishing epithelioid/biphasic mesothelioma from reactive mesothelial proliferations.