Deoxyribonuclease from beef pancreas, DNase I, was first crystallized by Kunitz. It is an endonuclease which splits phosphodiester linkages, preferentially adjacent to a pyrimidine nucleotide yielding 5'-phosphate terminated polynucleotides with a free hydroxyl group on position 3'. The average chain of limit digest is a tetranucleotide. DNase I acts upon single chain DNA, and upon double-stranded DNA and chromatin.

• DNase I is used as a genetic marker for forensic identification

One unit causes an increase in absorbance at 260 nm of 0,001/minute per ml at 25 °C, pH 5,0, when acting on highly polymerised DNA in the presence of ionised magnesium and calcium.