Cover photograph (Copyright © 2005, American Society for Microbiology. All Rights Reserved.): Scanning electron micrograph of wild-type blastocyst (embryonic day 3.5) cultured in vitro for 96 h. The inner cell mass (central region) is growing on the trophectoderm (outer region) formed by trophoblast giant cells in mouse embryos. ZPR1 deficiency caused embryonic death because of reduced cell proliferation and increased apoptosis. (See related article on p. 2744.)