(+)-Aphidicolin is a natural tetracyclic diterpene first isolated from the fungus C. aphidicola and shown to have antiviral activity against herpes simplex. In eukaryotic cells, it is a cell-permeable, reversible inhibitor of DNA replication, specifically blocking the activity of DNA polymerases α, δ, and ε when used at low micromolar levels. Aphidicolin can be used, at 3 μM, to arrest cells in G1/S phase or to increase gene amplification frequency.