Protease secretion of two CTC cell lines BHGc7 and BHGc10, derived from SCLC, is largely confined to MMP-9 and several members of the cathepsin family, with cathepsin S exclusively found in the CTCs and the metastatic and chemoresistant GLC16 cell line, but not in lines established from local metastases. This evidence concerns the gene MMP9 and small cell lung carcinoma.