Letter to the Editor High levels of mitochondrial DNA heteroplasmy in human hairs by Budowle et al.

The small size of the mitochondrial (mt) genome and its presence in many copies per cell make it useful for human DNA typing when only small amounts of material are available or when the material is badly degraded [1]. Despite its small size, and the fact that it is normally transmitted only maternally and without recombination, substantial DNA sequence polymorphism has accumulated in the mt genome, especially in two segments of the control region: HVI and HVII. The mtDNA sequence recovered from an item of evidence can thus be used to exclude many individuals as potential sources of that evidence. Conversely, if an evidence sequence matches a known reference sequence, the fre-quency of unrelated individuals who match the evidence sequence can be estimated by reference to a human mtDNA population database