Deficiency of fumarylacetoacetase without hereditary tyrosinemia

A variant of the enzyme fumarylacetoacetase (FAH) (E.C.3.7.1.2) in healthy individuals, determined by the enzyme activity, is reported. Analysis of family members of probands with low FAH activity suggests that the enzyme variant causing low activity could be the product of a pseudodeficiency gene. Assumed homozygotes for this gene have only slightly higher enzyme activity than patients with the metabolic disorder hereditary tyrosinemia I (hepatorenal type). No clinical abnormalities have been found in association with the postulated gene.