LINE-1 (L1) elements are retrotransposons that comprise large fractions of mammalian genomes. Transcription through L1 open reading frames is inefficient owing to an elongation defect, inhibiting the robust expression of L1 RNA and proteins, the substrate and enzyme(s) for retrotransposition. This elongation defect probably controls L1 transposition frequency in mammalian cells. Here we report bypassing this transcriptional defect by synthesizing the open reading frames of L1 from synthetic oligonucleotides, altering 24% of the nucleic acid sequence without changing the amino acid sequence. Such resynthesis led to greatly enhanced steady-state L1 RNA and protein levels. Remarkably, when the synthetic open reading frames were substituted for the wild-type open reading frames in an established retrotransposition assay, transposition levels increased more than 200-fold. This indicates that there are probably no large, rigidly conserved cis-acting nucleic acid sequences required for retrotransposition within L1 coding regions. These synthetic retrotransposons are also the most highly active L1 elements known so far and have potential as practical tools for manipulating mammalian genomes.