Mesothelin: A protein attached to the cell surface that is thought to have a role in cell-adhesion and possibly in cell-to-cell recognition and signalling. Mesothelin is so named because it is made by mesothelial cells. A monoclonal antibody, which recognises mesothelin, binds to the surface of cells from mesotheliomas and some other tumors but not to healthy tissues except for mesothelium. A blood test for soluble mesothelin-related proteins (SMR) can be used to identify people with mesothelioma and to monitor the progression of their disease. More patients with mesothelioma have raised concentrations of SMR than do those with other cancers or other inflammatory lung or pleural diseases. SMR concentrations also correlate with the size of the tumor and increase during tumor progression. The mesothelin gene MSLN is on chromosome 16.