A hexokinase is an enzyme that phosphorylates a six-carbon sugar, a hexose, to a hexose phosphate. In most tissues and organisms, glucose is the most important substrate of hexokinases, and glucose-6-phosphate the most important product. This phosphorylation provides the activation energy required for glycolysis. Hexokinase, therefore, enables glucose, taken up into muscle cells from the blood, to be used as an energy source for muscle actions. Hexokinase DEFICIENCY is an anemia-causing condition, the predominant clinical feature being non-spherocytic hemolytic anemia. This test is useful in the differential diagnosis of hemolytic anemia.