For a long time de novo molecular design algorithms for systematic and rational exploration of chemical space and quantitative structure-activity relationship (QSAR) modeling have been considered as tools that could broaden our horizons with less experimental costs and without the need to exhaustively evaluate as many as 10 33 possible drug-like compounds to find the few of interest. The relevance of QSAR modeling and de novo molecular design for drug discovery has been discussed many times , but these approaches can be just as useful in the areas of chemical biology that require new tool compounds and chemical probes that might not be constrained to drug-like molecules only .