BDNF and obesity due to melanocortin 4 receptor deficiency: These SNPs are all highly correlated, and although correlation was taken in to account in the MR model, the risk of biological pleiotropy (i.e., an effect of gene on the outcome not via smoking) was much harder to rule out; BDNF and its associated SNPs have reached genomewide significance in a number of other GWAS, including those of obesity and body mass index19, 20, and caffeine consumption21.