Busulfan is a chemotherapy drug that is an alkylating agent (slows the growth of cancer cells). It is marketed in the U.S. by Glaxo Smith Kline under the brand name Myleran, and has been in clinical use since 1959. Currently, its main uses are in bone marrow transplantation, especially in chronic myelogenous leukemia (CML), where it is used as a conditioning drug.