with the aim of maximizing sample size we have combined the clinical cases from the University of Edinburgh cohort with the community samples from QIMR and NTR. The extent to which this is important depends on the unknown genetic etiology of MDD. Lastly, 26% of both cases and controls contributed by the QIMR sample (which makes the largest contribution in this study) were from families ascertained on the basis of a sibling with nicotine dependence, as reflected by the high proportion of smokers in the QIMR samples. Smoking is associated with depression and if some genes are involved in smoking behavior and MDD, then the power to detect genetic variants for MDD may be reduced.
