sentences:  In humans, this enzyme is mainly expressed in the liver.FAH is among the amino acid hydroxylases.Tyrosine aminotransferase (TAT), 4-hydroxyphenylpyruvate dioxygenase (HPD), homogentisate 1,2-dioxygenase (HGD), phenylalanine-4-hydroxylase (PAH), maleylacetoacetate isomerase (GSTZ1),and other amino acid catabolic hydroxylases are coupled in the biological process of hydroxylases as well.