Chemical spaces lie at the heart of LBDD (see Sections 2.3 and 3.1 for a fuller discussion), but because of their representation dependence they are not unique. However, as noted earlier, this may not in many instances materially affect the effectiveness of ligand-based searches of chemical spaces . Maggiora has provided a relatively comprehensive discussion of molecular representations, similarity measures, and chemical spaces, which should be consulted for more details .