Cell storage tanks. View of a room of storage tanks for genetically engineered cells. These tanks are also incubators, keeping the cells at the optimum temperature for their propagation. Genetic engineering involves manipulating the DNA (deoxyribonucleic acid) of an organism to modify its function. DNA is the substance which controls every inherited trait and function of an organism. By finding which part of the molecule controls each function in the living organism, cells can be tailored to perform specific tasks. For example, some bacteria can be engineered to produce human insulin, which can then be used to treat diabetes.