Like cystatin C, immunoreactive cystatin M/E levels (median concentration into the pleural space: 0.14 nM) are significantly higher in effusions of primary pleural tumors (mesotheliomas) compared to secondary pleural tumors (3.4 μg/L vs. 2.5 μg/L) [22]. The gene discussed is CST6; the disease is pleural neoplasm.