Deficiency of this enzyme leads to the accumulation of upstream intermediates, including succinylacetone, a toxic metabolite traditionally regarded as pathognomonic for tyrosinemia type I. However, recent evidence has identified biallelic variants in the GSTZ1 gene as the molecular cause of maleylacetoacetate isomerase deficiency, a rare autosomal recessive condition that can mimic HT-1 in newborn screening due to mild or intermittent elevations of SA. The gene discussed is GSTZ1; the disease is Tyrosinemia type 1.