On Aug. 27, 1942, chemotherapy was first used to treat a cancer patient and the beginning of its critical role in the treatment of cancer. During World War II, naval personnel who were exposed to mustard gas during military action were found to have toxic changes in the bone marrow cells that develop into blood cells. During that same period, the US Army was studying a number of chemicals related to mustard gas to develop more effective agents for war and also develop protective measures.

Shortly after the discovery of nitrogen mustard, Sidney Farber in Boston demonstrated that aminopterin, a compound related to the vitamin folic acid, produced remissions in children with acute leukemia. That drug was the predecessor of methotrexate, a cancer treatment drug used commonly today. The era of chemotherapy had begun.