Another issue that concerns specialists is the acutal means of amplifying the mtDNA molecules. The PCR tests (polymerase chain reaction) are very easily contaminated. If a single molecule of modern DNA enters the reaction, the polymerase will preferentially amplify the modern DNA because it is so much younger and in better condition. Moreover, should the results of the test show strong comparisons of the ancient mtDNA and the reference sequence, the results are diagnosed as contaminated and they are excluded.

Statistical calculations of the average deviations from the reference sequence have provided results as follows:

• Modern human sequences differ among themselves by ~8.0±3.1 (range:1-24) substitutions.
• Neanderthal and human sequences differ by ~27.2±2.2 (range:22-36) substitutions.

As indicated, there is an overlap of the ranges for the farthest deviating human and the least deviating Neanderthal. This can be interpreted that there are H.sapiens sapiens that have mtDNA sequences closer to that of a Neanderthal than to other H.sapiens sapiens.