The last two decades have witnessed a remarkable burst of interest in the genetic diseases, with biochemical elucidations following close on clinical descriptions. The major problem for the investigator has usually been that of discovering a biochemical marker in order to focus more sharply on the site of the presumed genetically determined defect in a protein. Once the marker is found, it has not been difficult in most genetic disorders to narrow the range of pathogenetic possibilities sharply, using current knowledge of normal metabolic pathways in mammalian systems.
Cystine crystals were found in an autopsied child 70 years ago. Over the succeeding years this amino acid storage disease was clearly separated from cystinuria, a selective disorder of transport of specific amino acids in the kidney and the gut. The normal pathways for the metabolism of sulfur amino acids have been studied extensively. It would therefore seem to be a relatively