Many small molecule compounds with anticancer activities are discovered through phenotype-based screens. However, discovering the targets of these small molecules has been challenging. The gold standard for target identification requires the discovery of mutations in the target protein that block the effects of small molecules in vitro as well as in vivo. Here we describe the procedures for isolating drug resistant clones using the colorectal cancer cell line HCT-116 followed by whole-exome sequencing to identify recurrent mutations associated with compound resistance. Together with downstream in vitro and in vivo validation experiments, this strategy enables rapid target discovery for cytotoxic compounds.