Deoxyribonuclease I application is one unit will cause an increase in the absorbance at 260 nm of 0.001 per minute per ml at 25° C, pH 5.0 when acting on highly polymerized DNA according to the assay method of Kunitz (J.Gen.Physiol. 33, 349 and 363 (1950)). A solution in 1 mM Calcium Chloride and 50% (v/v) glycerol. Specific conditions for reactivity should be optimized by the end user.