In the early phases of embryonic development, extensive remodeling of condensed maternal and paternal gamete DNA occurs. The genes and related proteins responsible for chromatin reorganization are well characterized in vertebrates, such as Xenopus laevis, but have been unclear in mammals. In the April 25 Science, Kathleen Burns and colleagues at Baylor College of Medicine, Houston, show that nucleoplasmin 2 (NPM2), an oocyte-specific nuclear protein, is crucial in chromatin organization and post-fertilization development in mice (Science, 300:633-636, April 25, 2003).

Using Xenopus NPM2 as a template, Burns et al. isolated the murine NPM2 ortholog. In situ hybridization showed that expression of the NPM2 protein was limited to growing oocytes. To study the NPM2 activity, they used embryonic stem cell technology to generate homozygous NPM2-null mice. Although Xenopus NPM2 decondenses sperm DNA, the same process seems to proceed without NPM2 in mice, as NPM2-null males were...