The present invention relates generally to a method of modifying gene expression and to synthetic genes for modifying endo gene expression in a cell, tissue or organ of a transgenic organism, in particular a transgenic animal or plant. More particularly, the invention utilizes recombinant DNA technology to post-transcriptionally modify or modulate the expression of a target gene in tissue, organ or whole organism, thereby producing novel phenotypes. Novel synthetic genes and genetic constructs which are cap repressing delaying or otherwise reducing the expression of an endogenous gene or a target gene in an organism when introduced are also provided.