Transposons have been used in invertebrates for transgenesis and insertional mutagens in genetic screens. We tested a functional transposon called Sleeping Beauty in the one-cell mouse embryo. In this report, we describe experiments in which transposon vectors were injected into one-cell mouse embryos with mRNA expressing the SB10 transposase enzyme. Molecular evidence of transposition was obtained by cloning of insertion sites from multiple transgenic mice produced by SB10 mRNA/transposon coinjection. We also demonstrate germ-line transmission and expression from transposed elements. This technique has promise as a germ-line transgenesis method in other vertebrate species and for insertional mutagenesis in the mouse.