(2R-cis)-1-[2-(Hydroxymethyl)-1,3-oxathiolan-5-yl]-2,4(1H,3H)-pyrimidinedione (Lamivudine EP Impurity J) is an impurity of Lamivudine (L172500). (2R-cis)-1-[2-(Hydroxymethyl)-1,3-oxathiolan-5-yl]-2,4(1H,3H)-pyrimidinedione is also being further studied to determine its potential as an anti-HIV agent.