A 2,6-diaminopurine that is (1S)-cyclopent-2-en-1-ylmethanol in which the pro-R hydrogen at the 4-position is substituted by a 2-amino-6-(cyclopropylamino)-9H-purin-9-yl group. A nucleoside analogue reverse transcriptase inhibitor (NRTI) with antiretroviral activity against HIV, it is used (particularly as the sulfate) with other antiretrovirals in combination therapy of HIV infection.