Gene targeting by zinc-finger nucleases in one-cell embryos provides an expedite mutagenesis approach in mice, rats, and rabbits. This technology has been recently used to create knockout and knockin mutants through the deletion or insertion of nucleotides. Here we apply zinc-finger nucleases in one-cell mouse embryos to generate disease-related mutants harboring single nucleotide or codon replacements. Using a gene-targeting vector or a synthetic oligodesoxynucleotide as template for homologous recombination, we introduced missense and silent mutations into the Rab38 gene, encoding a small GTPase that regulates intracellular vesicle trafficking. These results demonstrate the feasibility of seamless gene editing in one-cell embryos to create genetic disease models and establish synthetic oligodesoxynucleotides as a simplified mutagenesis tool.