Cows have a protein in their white blood cells helps to make nitric oxide (a molecule very toxic to bacteria, including tuberculosis). It has been shown in other instances that if you have more of the protein (NRAMP1), then you tend to be more resistant to tuberculosis, and less of the protein, then you are less resistant to a tuberculosis infection. The protein itself is a transmembrane iron transporter. It just shuffles iron from one side of the membrane to another.<p>Cows already have the dna that encodes for NRAMP1, but in order to demonstrate their prowess with CRISPR technology, this group found a dormant location in the cow's genome in order to insert DNA that encodes a second copy of the same protein. The expectation being that by having a duplicate copy of the DNA that encodes for the NRAMP1 protein, more of the protein would be produced, and the cows would be less susceptible to tuberculosis. This seems to have been the case.<p>So here, adding DNA encoding a protein that already existed in the cow's genome allowed <i>more</i> of the protein (NRAMP1 - an iron transport machine) to be produced, thus conferring some tuberculosis resistance by means of allowing the cows white blood cells to produce nitric oxide more efficiently.