There are two different explanations for insulin production and they are contradictory. One says that the cDNA is inserted into the lac operon after the B galactosidase enzyme gene because they can share a promoter and this is the only way that a gene can be expressed. Without a promoter, a gene cannot be expressed. However, when they talk about identifying the recombinant plasmid the explanation changes. It says that a plasmid with 2 antiobiotic resistance genes is chosen and the insulin gene is placed in the middle of one of the antibiotic resistance genes to disrupt is function causing the other to act as a marker. Therefore the recombinant DNA can be identified when placed in each of the antibiotics. How can the antibiotic resistance method work when the gene is not placed in the lac operon? How can it be expressed without a promoter? Also how did the first method with the lac operon work? How was the recombinant plasmid identified in it?