Deoxyribonuclease I gene is approximately 3.2 kb long with 9 exons separated by 8 introns. In the form of a bovine pancreatic enzyme preparation, it occupies an important place in the history of protein chemistry and enzymology: it was the first enzyme to be recognized as specific for DNA; it was the first DNase to be crystallized; and it was the first DNase for which a specific protein inhibitor was characterized. DNase I is a Ca2+ and Mg2+ dependant endonuclease. DNase I is synthesized in the pancreas and stored in zymogen granules. It has been used to reduce the viscosity of cystic fibrosis sputum. A DNase I-like enzyme appears to catalyze the degradation of chromatin to oligo- and mononucleosomes during apoptosis. A recent study has demonstrated an endonuclease with activity and antigenicity indistinguishable from DNase I in thymocytes, cells susceptible to apoptosis. DNase I is an endonuclease that hydrolyzes double-stranded or single stranded DNA preferentially at sites adjacent to pyrimidine nucleotides. The product of hydrolysis is a complex mixture of 5'-phosphate mononucleotides and oligonucleotides. In the presence of Mg ion, DNase I attacks each strand of DNA independently and the cleavage sites are random.