Plasmids are circular DNA fragments within bacteria that replicate autonomously from the organism. As such, plasmids can be physically modified in order to put foreign strands of DNA in them, thus changing the code and subsequent phenotypal result of the plasmid. In order to insert the wanted gene into the plasmid, the plasmid must first be cut by a specific restriction enzyme that will only cut the plasmid in one location to prevent it from destroying itself. This restriction enzyme will form a specific pattern of sticky ends on the cut plasmid. It is important to note that every restriction enzyme will code to a different sticky end and that each set of sticky ends are complementary to only each other. After that, the restriction enzyme will cut the DNA with the same sticky end pattern on both sides of the wanted gene, and insert the gene into the plasmid, sticking the ends together using the enzyme ligase, thus once more making the plasmid circular. A simple, modular method for determining which bacterial cells have been successfully transformed is to, regarding the type of gene used, specifically apply certain substances that with the influence of the gene will be modified and constructed into another substance, or to inflict a certain condition or status that can be detected by and reacted to by a specific gene and its derivatives. To do this, however, it is necessary to know exactly what the original gene will code for. After applying the certain status or substances, it is then simple to test which cells produce product or react, and which do not.