Inherited metabolic diseases generate research activity of far greater intensity than one might expect from their rela-tively rare occurrence. This is because genetic disorders afford a unique op-portunity to combine the concepts of genetics with the tools of biochemistry to study the metabolism of man, as has been so successfully done for the metabolism of microorganisms. The les-son of genetics is clear: genes contain the code for the structure of proteins; a mutation in a gene will result in an alteration of the specific protein to which that gene holds the code. The result may be benign or disastrous, depending on the importance of the protein to the overall metabolism and on the effect of the structural change on its function. Faced with a disease of genetic origin, the biochemist's task is to identify the altered protein which is specific to that disorder. Success may lead both to practical applications in the managementof the disease and to a clearer understanding of normal meta-bolic processes.