The major limitation of our study is the potential susceptibility of the case-control design to population stratification, especially because our controls originated from 3 different groups that did not closely correspond in their geographic origin to our proband sample. While we addressed this issue by matching self-reported ethnicities of probands and controls, spurious effects in case-control studies (which are regarded by some authors as relatively uncommon49) can be ruled out by genotyping multiple ancestry-informative markers, which was not feasible for this work. Dense genetic information on SLC1A1 markers from different ethnicities and geographic locations will increasingly become available from genome-wide association studies. Such data will allow a better judgment of how strongly (and if at all) population stratification is relevant to the SLC1A1 markers analyzed herein.
