Fazarabine, also known as Kymarabine, is an orally-active pyrimidine analogue of an aza-substituted cytidine in which the ribose moiety is replaced by an arabinose sugar. Similar in action to cytarabine, fazarabine is phosphorylated by deoxycytidine kinase to a triphosphate form which competes with thymidine for incorporation into DNA; its incorporation into DNA inhibits DNA synthesis, resulting in tumor cell death and tumor necrosis. The presence of deoxycytidine kinase in a tumor is a determinant of tumor sensitivity to this drug.