Genomic DNA sequencing and alignment with the known DNase I mRNA showed that the bovine gene consists of 9 exons and that only the last 8 encode the protein, since initial ATG was found at exon II. RT-PCR was used to identify DNase I mRNA in lens epithelium in vivo and in cultured epithelial cells. We found DNase I transcripts having the same nucleotide sequence as the pancreas form and others lacking almost all exon V. The lens protein presented a slightly higher relative molecular weight than the pancreatic enzyme. Lens DNase I was located in secretory pathway organelles and excluded from the nucleus. Nevertheless, in apoptotic lens epithelial cells in vitro, DNase I translocated to the nucleus and co-localized with TUNEL positive chromatin aggregates. These results indicate that cells in the lens epithelium constitutively express DNase I, and suggest a direct involvement of this nuclease in the final phases of chromatin degradation. (C) 2003 Elsevier Inc. All rights reserved.