The relationship appeared to hold true for those cell lines derived from lung epithelial primary tumors but not for the neuroendocrine/small-cell and mesothelioma cell lines. siRNA knockouts to DDH-1 and DDH-2 were prepared with the cell line exhibiting the greatest resistance to cisplatin (A549) resulting in marked decreases in the DDH isoforms as assessed by real-time PCR, western blot and enzymatic activity.