PDPN and lip and oral cavity carcinoma: Podoplanin (PDPN), the endogenous ligand of C-type lectin-like receptor-2 (CLEC-2) [1,2], is highly expressed not only in various tumors including oral cancer, lung cancer, esophageal cancer, malignant brain tumors, mesotheliomas, testicular tumors, and osteosarcoma [3–13], but also in normal cells such as lymphatic endothelial cells and podocytes [14,15].