Threshold expression

Threshold expression is a phenomenon in which phenotypic expression of a mitochondrial disease within an organ system occurs when the severity of the mutation, relative number of mutant mtDNA, and reliance of the organ system on oxidative phosphorylation combine in such a way that ATP production of the tissue falls below the level required by the tissue. The phenotype may be expressed even if the percentage of mutant mtDNA is below 50% if the mutation is severe enough.