N6-Cyclopropyl-9-[(1R,4S)-4-[(1,1-dimethylethoxy)methyl]-2-cyclopenten-1-yl]-9H-purine-2,6-diamine-d4 is deuterium labeled N6-Cyclopropyl-9-[(1R,4S)-4-[(1,1-dimethylethoxy)methyl]-2-cyclopenten-1-yl]-9H-purine-2,6-diamine (C989915), an impurity of Abacavir (A104990), which is a guanosine analog that inhibits reverse transcriptase (NRTI)(1,2).