Thapsigargin is a compound found in a plant that inhibits a calcium pump known as SERCA. It has been used as a tool for studying intracellular calcium regulation, protein unfolding, and cell death. Thapsigargin has also been chemically transformed into a prodrug for cancer treatment. However, the drug failed clinical trials, but the chemical synthesis of thapsigargin led to important scientific discoveries. This finding includes new understandings of sesquiterpene lactones, ATPase mechanisms, prodrug development, and plant taxonomy.