Bacteria are used by industry in various ways. Some vaccines are made of attenuated pathogenic bacteria or antigens present in bacteria. One of the most ancient uses of bacteria is the fermentation of milk to produce yogurt, cheese and curds (even before people knew of the existence of bacteria, these microorganisms were already used to make those products). Some methods of antibiotic production involve bacteria. Recombinant DNA technology (genetic engineering) allows the industrial production and commercialization of human proteins, such as insulin for diabetics, which is synthesized by mutant bacteria. Some bacteria can produce fuel, like methane gas.