Hereditary tyrosinemia type 1 (HT1) is an autosomal recessive disorder affecting fumarylacetoacetate hydrolase (FAH), the last enzyme in the tyrosine catabolism pathway. The liver mosaicism observed in HT1 patients is due to the reversion to the wild type of one allele of the original point mutation in fah. It is generally accepted that these reversions are true back mutations; however, the mechanism is still unresolved. Previous reports excluded intragenic recombination, mitotic recombination, or homologous recombination with a pseudogene as possible mechanisms of mutation reversion in HT1. Sequence analysis did not reveal DNA motifs, tandem repeats or other sequence peculiarities that may be involved in mutation reversion. We propose the hypothesis that a point mutation instability mutator (PIN) phenotype brought about by the sustained stress environment created by the accumulating metabolites in the cell is the driver of the true back mutations in HT1. The metabolites accumulating in HT1 create a sustained stress environment by activating the extracellular signal-regulated kinase (ERK) and AKT survival pathways, inducing aberrant mitosis and development of death resistant cells, depleting glutathione, and impairing DNA ligase IV and possibly DNA polymerases δ and ε. This continual production of proliferative and stress-related survival signals in the cellular environment coupled with the mutagenicity of FAA, may instigate a mutator phenotype and could end in tumorigenesis and/or mutation reversion. The establishment of a PIN-mutator phenotype therefore not only seems to be a possible mechanism underlying the true back mutations, but also contributes to explaining the clinical heterogeneity seen in hereditary tyrosinemia type 1.