Inborn errors of metabolism are genetic diseases--generally exhibited at birth or during childhood--caused by the absence or imbalance of one or more enzymes necessary in normal development and growth. There is extensive chronic illness, mental retardation, and premature death associated with many of these disorders. Our laboratory has chosen to focus its research efforts on certain inborn errors of metabolism, especially cystinosis and other diseases involving aberrations of sulfur amino acid metabolism. We have recently discovered the enzyme deficiency responsible for one of these diseases, glutathionuria, which is due to extreme tissue deficiency of the enzyme gamma-glutamyl transpeptidase. In other disorders, the cause of the metabolic derangement is still obscure, and our attention is directed toward the elucidation of the primary biochemical derangement. Our laboratory also is attempting to develop new drugs for the treatment of genetic diseases and is studying the consequences of newer therapeutic measures now considered experimental. Another major concern is the development of improved methods of early diagnosis for genetic derangements, including antenatal diagnosis which may be of preventive value, and the provision of genetic counselling to the community.