Immunofluorescence staining for podoplanin (red) and cytokeratins (green) in malignant serosal tumors morphologically consistent with sarcomatous mesothelioma. In A, the lining mesothelium (solid arrows) stained positively for podoplanin in this double label immunofluorescent image of diaphragm. The cells beneath the mesothelial lining are also red due to expression of podoplanin and these are cells of a malignant serosal tumor. In B, only the single label green fluorescence is shown to demonstrate weak expression of cytokeratins in the lining mesothelium (solid arrow), while staining of the subjacent tumor for cytokeratins is equivocal. In C, a double label immunofluorescent image demonstrates a malignant serosal tumor between the liver and gall bladder that is lined by reactive mesothelium (solid arrow) which stains red for podoplanin as well as green for cytokeratins. The cells of the subjacent malignant serosal tumor stain weakly red for podoplanin. In D, the photomicrograph shows this same tumor but only the green fluorescence for cytokeratins. The reactive mesothelium lining the malignant serosal tumor (solid arrow) stains green for cytokeratins while the serosal tumor has no evidence of cytokeratin expression. The normal mesothelium lining the liver is weakly positive for cytokeratins (dashed arrow). The epithelium lining the gall bladder (open arrow) strongly expresses cytokeratins. Magnification bar is 50 μm.