Leto creates DNA sequences optimized for the expression in target hosts, from any available DNA or protein sequence. It performs a multi-objective optimization, taking into account such parameters as codon usage, mRNA secondary structure, direct repeats, cryptic splice sites and regulatory motifs. It pursues a simultaneous multi-objective optimization; a genetic algorithm searches for a sequence that is optimal with respect to all relevant parameters. This prevents the circumstance in which the earlier optimization of one parameter is overridden by the subsequent optimization of other parameters. The software has an extensive GUI for editing and annotating the optimized sequence and can be run on many operating systems including Windows, Mac OS X and Linux.