EC 5.3.1.5 covers the xylose isomerases, enzymes that interconvert D-xylose and D-xylulose and also isomerise D-ribose and D-glucose. Their fame rests on one reaction: turning glucose into fructose for high-fructose corn syrup, among the largest enzyme applications in the food industry, where immobilised-enzyme columns convert glucose syrups continuously. The industry still widely calls them glucose isomerases. The same enzymes let engineered yeasts ferment wood-derived xylose to ethanol by routing it through xylulose, connecting them to biofuel work. Their toughness under plant conditions made enzymatic isomerisation a landmark of food biotechnology.