Determining mechanisms of drug action in human cells remains a major challenge. Here we describe an approach in which multiple-drug-resistant clones are isolated and transcriptome sequencing is used to find mutations in each clone. Further analysis of mutations common to more than one clone can identify a drug's physiological target and indirect resistance mechanisms, as indicated by our proof-of-concept studies of the cytotoxic anticancer drugs BI 2536 and bortezomib.