Mutant forms of two enzymes of the pentose phosphate pathway have been associated with disease in humans. A mutation in ribose-5-phosphate isomerase (RPIA), which normally mediates the reversible interconversion of D-ribulose-5-phosphate and ribose-5-phosphate, has been associated with a slowly progressive leukoencephalopathy, and mutations in transaldolase 1 (TALDO1), which normally mediates the reversible interconversion of D-fructose 6-phosphate and D-erythrose-4-phosphate to form sedoheptulose-7-phosphate and D-glyceraldehyde-3-phosphate, have been associated with congenital liver disease (Wamelink et al. 2008).