Aphidicolin is a tetracyclic diterpene antibiotic first isolated from the fungus Lecanicillium muscarium with antiviral and antimitotic properties. Aphidicolin is a reversible inhibitor of eukaryotic nuclear DNA replication. It blocks the cell cycle at early S phase. It is a specific inhibitor of DNA polymerase Alpha and Delta in eukaryotic cells and in some viruses and an apoptosis inducer in HeLa cells. Natural aphidicolin is also a secondary metabolite of the fungus Nigrospora oryzae.