Every characteristic of human anatomy and physiology is
determined by biochemical reactions catalysed by enzymes.
These in turn are determined by our genetic make-up. If a
gene is defective or missing it will result in a defective or
missing enzyme—a so-called Inborn Error of Metabolism.
In a pathway a → b → c → d, if the enzyme that
converts c to d is defective, or missing, d cannot be made
and c may accumulate so that abnormal concentrations
may build up—perhaps in the blood or urine. Knowledge
of the defective enzyme and its gene(s) may make
possible the rational treatment of the resulting disease.
The Inborn Errors of Metabolism map identifies the sites
of 40 of the most important metabolic deficiency diseases which
are named on the sides of the map.