Cytosine normally combines with a different sugar, deoxyribose, to form deoxycytidine, a component of DNA.
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Its mode of action is due to its rapid conversion into cytosine arabinoside triphosphate, which damages DNA when the cell cycle holds in the S phase (synthesis of DNA).
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It was approved by the United States Food and Drug Administration in June 1969, and was initially marketed in the U.S. by Upjohn under the trade name Cytosar-U.