A classic example is nitisinone (OrfadinTM, Sobi) for the treatment of hereditary tyrosinaemia type 1 (HT-1, OMIM 276700) [14], an IEM caused by deficiency of fumarylacetoacetate hydrolase (FAH) which catalyses the sixth enzymatic step of tyrosine degradation (Figure 1B). The gene discussed is FAH; the disease is hematocrit.