Another cancer stem cell marker of malignant mesothelioma, CD9, inhibits this metastatic effect mediated by CD26. Depletion of CD26 and CD9 was shown to respectively lead to decreased and increased expression of NEDD9 and FAK in mesothelioma cells lines, hinting at the involvement of NEDD9 in mesothelioma tumor invasiveness [91]. Here, PTK2 is linked to mesothelioma.