HPD and tyrosinemia: Tyrosinemia is an inherited disorder in a single pathway involving mutations in one of three distinct enzymes involved in tyrosine degradation—fumaroylacetoacetate hydrolase (EC 3.7.1.2) (Grompe et al., 1994), TAT (James et al., 2005) or 4-hydroxyphenylpyruvate dioxygenase (Hpd) (EC 1.13.11.27), the last of which is very rare.