Features selected by mRMR+FFS for the four different groups are quite different from each other, showing the intrinsic differences between them. Although there are more features for target than those for drug in the original feature set, more drug features were selected, showing the important role of drugs. Many of the selected target features are for protein secondary structure, especially for enzyme group (half of selected target features are for this). All types of features are selected in at least one group, showing that all biochemical and physicochemical features have their irreplaceable positions in drug-target interaction process.