Microheteroplasmy
In biology, microheteroplasmy is a form of heteroplasmy, a type of mutational damage to mitochondrial DNA. While heteroplasmy in general includes mutations present in any fraction of mitochondrial DNA (from fractions of a percent to almost a hundred percent), microheteroplasmy is the presence of mutations levels of up to about 2−5% of mitochondrial genomes. In human mitochondrial DNA, microheteroplasmy constitutes hundreds of independent mutations in one organism, with each mutation usually found in 1–2% of all mitochondrial genomes. The distinction of microheteroplasmy and more gross heteroplasmy is dictated by technical considerations - classical DNA sequencing of mitochondrial DNA by the  use of PCR is capable only of detecting mutations at levels of 10% or more, as a result of which mutations at lower levels were never systematically observed until the work of Lin et al. As it became apparent after the use of Lin's cloning and sequencing strategy, capable of detecting mutations at levels of 1% or less, such low-level heteroplasmy, or microheteroplasmy, is exceedingly common, and is in fact the most common form of mutational damage to human DNA found to date. In aged adults, each copy of mitochondrial DNA has on average 3.3 mutations changing protein  structure. This exceeds previous estimates by more than three orders of magnitude. The discovery of microheteroplasmy lends support to the mitochondrial theory of aging, and has already been linked to the causation of Parkinson's disease.