[8] As a critical enzyme in phenylalanine and tyrosine metabolism,  4-Fumarylacetoacetate hydrolase catalyzes the final step in the catabolism of 4-fumarylacetoacetate and water into acetoacetate, fumarate, and H+ respectively.

[11] Similar to Phenylalanine-associated pathways, the reaction molecular basis is critical in mammalian metabolism, as evidenced by the observed liver enzyme activity in FAH deficiency during hereditary tyrosinemia type 1.

[15] As an inborn error of metabolism, Tyrosinemia type I stems from a deficiency in the enzymatic catabolic pathway of fumarylacetoacetate hydrolase (FAH).

[18][14] However, symptoms may appear in heterozygote mutations in the FAH gene as documented in case of a 12‐year‐old American boy with chronic tyrosinemia type 1.

[18] This is also attributed to observed clustering between amino acid residue active sites 230 and 250 among hundreds of other mutations in the FAH gene.

[15] Caused by the lack of fumarylacetoacetate hydrolase (FAH), the last enzyme of the tyrosine catabolic pathway, HT 1 is inherited as a rare autosomal recessive disease with a prevalence in Europe of 1 : 50000.

[15][18] However, in isolated parts of Quebec's provinces, the frequency can be as high as 1 : 2000 with a carrier rate of 1:20 possibly due to a single founder mutation.

[20] The chronic form has an age of onset of more than one year after birth;[21] rickets and progressive liver disease often lead to the development of hepatocellular carcinoma.

Bacterial inhibition assay, such as the Guthrie Test, can screen newborns[21] for FAH deficiency in addition to increased phenylalanine levels.

The identification of the gene defects on both alleles enables an initial genotype-phenotype analysis for chronic, subacute and acute HT 1 patients.