Aspects of the invention relate to the design and synthesis of nucleic acid libraries containing non-random mutations or variants. Aspects of the invention provide methods for assembling libraries containing high densities of predetermined variant sequences. Certain embodiments relate to the design and synthesis of nucleic acid libraries that express a predetermined polypeptide from a library of nucleic acids having silent sequence variants. Certain embodiments relate to the design and synthesis of nucleic acid libraries that express predetermined RNA variants that encode the same polypeptide sequence.
