With respect to identification of population substructure there are several limitations in the current study. First, analyses are based on a diverse set of individuals of European descent with variable ancestral contributions from different European countries that is only partially defined. This limits certain conclusions with regards to specific aspects of substructure related to population subgroups. However, we believe that the concordant grouping of the majority of participants with grandparental information provides strong support for the major relationships and differences in these population groups. The overall strong correlation between results using principal components and those using a Bayesian clustering algorithm provide additional confidence in the general results. Second, the PCA is sensitive to differences in the inclusion or exclusion of specific population groups. When the second axis is considered for the entire European group, we observed changes in the country-of-origin order for the northern group with respect to the southern group in subset analyses (Figure 2). We speculate that this observation may reflect the difference in the origins of the additional substructure in the northern group compared to the other
