Recombination of insertion modified nucleic acids

A method of making a nucleic acid with a desired splicing phenotype, the method comprising:

• providing a plurality of homologous nucleic acids, each comprising a plurality of insertion nucleic acid sequences, wherein the plurality of insertion nucleic acid sequences comprise heterologous inteins;
• recombining the plurality of homologous nucleic acids to produce a library of recombinant nucleic acids, and
• selecting the recombinant nucleic acids for a selected recombinant nucleic acid which produces a desired mRNA or protein possessing the desired splicing phenotype when the selected recombinant nucleic acid is expressed in a cell.