Saccharopinuria, also known as hyperlysinemia type II, is an autosomal recessive metabolic condition with few, if any, clinical manifestations. Hyperlysinemia type II and hyperlysinemia type I (238700) both result from deficiency of the bifunctional enzyme AASS (605113) on chromosome 7q31. The AASS gene encodes lysine alpha-ketoglutarate reductase and saccharopine dehydrogenase, which catalyze, respectively, the sequential conversion of lysine to saccharopine and saccharopine to alpha-aminoadipic semialdehyde and glutamate (summary by Tondo et al., 2013). In hyperlysinemia type I, both enzymatic functions of AASS are defective and patients have increased serum lysine and possibly increased saccharopine; in hyperlysinemia type II, most of the first enzymatic function is retained, and patients tend to have isolated saccharopine increase (Cox, 1985; Cox et al., 1986).