An acyclic analog of the nucleoside guanosine, chemically related to acyclovir. It inhibits the replication of viruses including CMV, Herpes Simplex, Varicella Zoster and Epstein-Barr. It inhibits virus replication by its incorporation into viral DNA. This incorporation inhibits dATP and leads to defective DNA, ceasing or retarding the viral machinery required to spread the virus to other cells.