Response to letter to the editor: Comment on "Discriminating toxicant classes by mode of action: 3. Substructure indicators" (M. Nendza and M. Müller, SAR QSAR Environ. Res. 18 155 (2007)) (P. C. von der Ohe, R. Kühne, R.U. Ebert and G. Schüürmann, SAR QSAR Environ. Res. 18 621 (2007))