Cell death by apoptosis is involved in a large variety of developmental events and physiological processes requiring a reduction in cell count. Nuclear collapse, one of the first visible changes denoting irreversible commitment to cell death by apoptosis, is frequently accompanied by chromatin degradation into nucleosome-sized fragments of multiples thereof. The identity of the endonuclease responsible for this DNA digestion has attracted some interest in recent years and several candidate endonucleases have been proposed. The scope of this article is to summarise the present knowledge about deoxyribonuclease I, one of the candidate enzymes.