Herpes virus ribonucleotide reductase, which consists of two nonidentical subunits, designated R1 and R2, is an essential enzyme responsible for reduction of four ribonucleoside diphosphates to corresponding deoxyribonucleoside diphosphates, and it is the rate-limiting step in DNA synthesis. The R2 subunit contains a tyrosylfree radical and a binuclear m-oxo bridged iron center, which are required for ribonucleotide reductase activity. The characters of herpes viruses R2 gene and protein are described in the present article.