[A synthetic cyclopentenyl antimetabolite analogue of cytidine with antineoplastic activity and antiviral activity. Cyclopentenyl cytosine competitively inhibits cytidine triphosphate (CTP) synthase, thereby depleting cytidine pools and inhibiting DNA synthesis. This agent may also induce differentiation of some tumor cell types. Cyclopentenyl cytosine's antiviral activity is broad-spectrum. (NCI04) ( NCI )]