1-(2-Deoxy-2-fluoro-β-D-arabinofuranosyl)-5-bromocytosine, also known as 4-amino-5-bromo-1-[(2R,3S,4R,5R)-3-fluoro-4-hydroxy-5-(hydroxymethyl)oxolan-2-yl]pyrimidin-2-one, is a fluorinated nucleoside analog. Its structure features a cytosine base modified with a bromine atom at the 5-position, linked to a 2'-deoxy-2'-fluoro-β-D-arabinofuranosyl sugar moiety. This specific configuration of fluorine and hydroxyl groups on the sugar ring is characteristic of the arabino configuration. This compound is a key synthetic intermediate in the preparation of antiviral and anticancer nucleoside drugs. Most notably, it serves as a crucial precursor in the synthesis of **Clevudine (L-FMAU)**, a potent antiviral agent used for the treatment of chronic hepatitis B virus (HBV) infection. The bromine atom at the 5-position of the cytosine base provides a reactive handle for further chemical modifications, enabling the introduction of other functional groups necessary to complete the drug molecule. As a fluorinated and halogenated nucleoside derivative, it falls under the category of modified nucleosides, which are essential building blocks in medicinal chemistry for developing therapeutics that target viral polymerases and cellular enzymes involved in nucleic acid metabolism.