Described herein are synonymously altered gene sequences which express protein in differing levels within secretory as compared to non-secretory target tissue. An expression cassette comprising an open reading frame (ORF) for a protein under the control of regulatory sequences which direct expression of the product in cell, which ORF has been modified to preferentially increase expression levels in a selected tissue, wherein the modified ORF is characterized by a triplet frequency of any one of Tables 3-12, 16 or 17.
