Monocarboxylate such as lactate and pyruvate play an important role in cellular metabolism. Lactic acid is produced as the end product of glycolysis. Some tissues, such as white skeletal muscle and, red blood cells, use this pathway to generate most of their ATP under normal physiological conditions. All tissues become dependent on this pathway during abnormal conditions such as hypoxia and ischaemia. Lactic acid, produced during normal glycolysis, must be transported out of cells to sustain maintain high rate of glycolysis. Failure to export lactic acid leads to accumulation of cellular lactic acid followed by an increase in pH and inhibition of glycolysis. Some tissues, such as brain, heart, and red skeletal muscle, readily oxidize lactic acid, and must import lactic acid into the cells. Lactic acid transport is mediated by a group of proton-linked membrane transporters called monocarboxylic acid transporters (MCTs). At least 9 MCT-related proteins (MCT1-9) have been identified in mammals that are expressed in a tissue specific manner. MCT8 or XPCT (X-linked PEST Containing Transporter; (mouse, 545 aa, human 613/598 aa, chromosome xq13.2) is highly expressed in liver, heart, and kidney.