Positive and negative contribution of some fragments towards P-gp antagonism, obtained by HQSAR analysis. Grey = carbon atoms, where C2, C3 and Car indicates sp2, sp3 and aromatic carbon. Red = oxygen atom, where O2 and O3 indicates sp2 and sp3 oxygen, respectively. Blue = nitrogen atom, where N3, Nar indicates sp3 and aromatic nitrogen. Blue = ANY atom, and it may be hydrogen, carbon or oxygen.