BMC Cancer 2009, 9:113 http://www.biomedcentral.com/1471-2407/9/113 database does not mean that they are mutational hotspots, but rather reflects the inheritance of the corre- sponding mutational variant in European matrilines. For instance, no position in the region 4527–4954 would qualify as a mutational hotspot with more than three mutations in the mtDNA tree of Kivisild et al. [21]. The nucleotide variant located in this DNA fragment that is by far most frequently found in Europe is the nucleotide inherited from the transition A4917G characterizing hap- logroup T, which has frequency ~10% across Europe. Next in frequency (~5%) comes G4580A in West Europe, char- acteristic of haplogroup V. Now, if one would randomly permute paired identical sequences, that is, deliberately generate mosaic assignments, then the typical sequence contrast would involve position 4917 and to a lesser extent 4580 and finally positions 4529, 4769, and 4793. Now, the variation recorded in Table 1 of [11] mirrors exactly such hypothetical sample mixing: four events at 4917, which could reflect perfect sample crossover (since two changes are A to G and two are G to A), whereas one event is at 4580 and another one at 4769! Thus, it is very unlikely that the recorded haplotypic variation presents authentic somatic variation, which would be expected to hit positions from the entire fragment randomly, and not just affect those few that signify major European haplo- groups. The fragment 30–407, which encompasses the so-called second hypervariable segment of the control region, har- bours a number of extreme mutational hotspots. The top- most changes are the transitions at positions 146, 150, 152, 195, and the length polymorphism of the C stretch preceding position 310, which is often found in hetero- plasmic state in individuals, healthy or not. This entails that transitional changes at positions 146, 150, 152, and 195 can be found in almost all combinations (haplo- types) among samples from the general population. In contrast, relatively rare changes are, for instance, the trans- version C186A and the transitions C186T, A240G, C285T, and C295T, where the latter two variants are excellent markers for haplogroups U1 and J, respectively. The salient feature of the changes recorded in Table 2 of Prior et al. [11] is that whole arrays of mutations (such as the motif T146C-T152C-C186A), involving several hyper- variable as well as single stable sites, switch back and forth. Such a concerted mutational process has never been observed in mtDNA population studies other than in cases of artificial recombination of amplicons [6,24]. One can, for instance, interpret the mtDNA alterations in Patient 10 as the result of sample mix between a genuine haplogroup V mtDNA (testified by G4580A, and carrying a rare private change A240G) and a haplogroup J1c sequence (indicated by G228A and C295T). We conclude that virtually all the mtDNA alterations recorded in the study of Prior et al. [11] can be perfectly explained by sample mixing and contamination. The observed pattern itself is incompatible with features of a natural mutational process that would never change linked complex arrays of point mutations back and forth. Buccal cells The study by Tan et al. [12] reported numerous mtDNA alterations in buccal cells from smokers versus non-smok- ers. The tables in this paper display only the cumulated differences between mtDNAs in lymphocytes and in buc- cal cells within the cohort of smokers and that of non- smokers, respectively, but do not provide the information about the set of alterations (haplotypes) in every single individual. Such haplotype information is usually neces- sary for critical reading and interpretation of the experi- mental results since it directly hints at potential artifacts. In this case, however, the surprisingly large number of seeming homoplasmic somatic changes, although just cumulatively listed in their Table 3, is quite telling because a number of mutations are known to occur together on pathways of the mtDNA phylogeny. Specifically, the nine mutations at sites 10086, 10373, 10398, 13105, 15824, 15944del, 16223, 16278, and 16362 unambiguously determine a haplogroup L3b1 lin- eage α, the seven mutations at 10115, 10530, 10398, 13590, 13650, 16223, and 16278 signify a particular mtDNA lineage β belonging to haplogroup L2c [21], and the five mutations at 4529, 10034, 10238, 10398, and 16223 indicate a lineage γ of haplogroup I status. The presence of these lineages in the samples analyzed by Tan et al. [12] (see their Table 3) gives a strong signature of contamination or sample mix-up. Indeed, among 2704 entire coding region sequences stored in the mtDB [14], T10115C exactly identifies the 61 haplogroup L2 sequences, A4529T the 33 haplogroup I sequences, and A15824G the 17 haplogroup L3b sequences – without exception. These mutations are thus excellent haplogroup markers on their own. The fact that even 3–5 additional coding region mutations (plus further mutations from the more variable control region) come in support of the respective haplogroup profile renders the haplogroup sig- nature bullet-proof. In addition, one could further postulate the presence of a particular haplogroup T2b lineage δ bearing the four mutations at 930, 9947, 13563, and 16294 [25], although none of the mutations alone would be characteristic of this lineage. In fact, G9947A would also enter the motif of haplogroup F3b and C13653T that of haplogroup G2. With respect to the amplicon range "Dloop1" (16100– 16544, including primer locations) [26], the four muta- tions at A16220C, A16265G, T16298C, and T16362C Page 3 of 8 (page number not for citation purposes)