The invasion and metastasis of colorectal, breast and lung cancers has been associated with the overexpression of cysteine protease inhibitor SN (Cystatin SN, CST1).137 Clinical data indicate that CST1 levels are elevated in both peripheral blood and ascites in gastric cancer patients with metastasis, and CST1 expression is significantly elevated in metastatic gastric cancer tissues. The gene discussed is CST1; the disease is lung cancer.