Minichromosome maintenance 10 (MCM10) is a conserved, abundant nuclear protein, which plays a key role in the initiation of eukaryotic chromosomal DNA replication and elongation. To elucidate the physiological importance of MCM10 in vivo, we generated conventional knockout mice. No MCM10-null embryos were recovered after E8.5, and the mutation was found to be lethal before the implantation stage. Mutant embryos showed apparently normal growth until the morula stage, but growth defects after this stage. The dramatic reduction of 5-bromo-2-deoxyuridine (BrdU) incorporation in the mutant embryo, followed by cell death, suggests that defective cell proliferation may underlie this developmental failure. Taken together, these findings provide the first unequivocal genetic evidence for an essential and non-redundant physiological role of MCM10 during murine peri-implantation development.