Many problems and stumbling blocks must be overcome before the product of a foreign gene   can be manifactured in microorganisims.First,the correct regulatory sitse must be positioned at the beginning of the cloned DNA fragment to ensure correct transcription and translation of the gene.However, even if the gene is correctly transcribed and translated , cells may recognize the products of the foreign gene as foreign proteins and destroy them before they can be isolated.And the recently discovered fact that eukaryotic DNA is recognized as split genes whit introns and exons means that genes cloned directly from human DNA cannot be translated succesfully in microorganisims because they do not have the necessary RNA splicing enzymes that are present in animals.