((2R,5S)-5-(4-Amino-5-fluoro-2-oxopyrimidin-1(2H)-yl)-1,3-oxathiolan-2-yl)methyl hydrogen ((((R)-1-(6-amino-9H-purin-9-yl)propan-2-yl)oxy)methyl)phosphonate is an impurity of Emtricitabine (E525000), a reverse transcriptase inhibitor. It is an effective antiviral agent against HIV, HBV, and other viruses replicating in a similar manner. A nucleoside analog structurally related to Lamivudine (L172500).