Deoxyribonuclease I (DNase I) is a Ca2+ and Mg2+ dependent 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.