Benzonase is a genetically engineered endonuclease from Serratia marcescens (1, 2). The enzyme is produced and purified from E. coli strain W3110, a mutant of strain K12, containing theproprietary pNUC1 production plasmid (3, 4). Structurally, the protein is a dimer of identical 245amino acid, ~30 kDa subunits with two essential disulfide bonds (5–8). This promiscuousendonuclease attacks and degrades all forms of DNA and RNA (single stranded, double stranded,linear and circular) and is effective over a wide range of operating conditions (9). The enzyme completely digests nucleic acids to 5′-mono phosphate terminated oligonucleotides 2–5 bases in length (2, 10). Although the nuclease is capable of cleavage at nearly all positions along a nucleic acid chain, sequence-dependent preferences have been demonstrated (11). The enzyme prefersGC-rich regions in dsDNA while avoiding d(A)/d(T)-tracts. Benzonase is now available from Novagen in two grades, Purity > 99% and Purity > 90%. Both preparations possess exceptionally high specific activities and are supplied free from measurable protease activities and viral contaminants. Benzonase is ideal for a wide variety of applications where complete digestion of nucleic acids is desirable.