The authors then examined expression of these genes in postmortem brain tissue collected from 51 pairs of human subjects, with each pair consisting of one subject with major depressive disorder and one comparison subject with no psychiatric disorders, in order to determine whether disruption of the mechanisms they found in mice also exist in individuals with major depression. The pairs were matched for sex, with nearly identical group means for postmortem delay, brain pH, and RNA quality. The tissue samples were of high quality in terms of these three latter factors, all of which play critical roles in determining the validity of gene expression studies. The authors observed a 30% reduction in the expression of TRKB, but not BDNF, in the subgenual anterior cingulate cortex in depressed subjects. Significant decreases were also detected in the expression of genes with high (CORT, VGF, SST, and NPY), intermediate (SNAP25 and GAD2 [GAD65]), and low (GAD1 [GAD67] and PVALB [PV]) levels of BDNF dependency. When the sample was segregated by sex, male subjects exhibited more robust differences in expression, which may have been related to sex-dependent expression differences in comparison subjects. The size of the cohort also allowed the authors to rule out potential confounds of medication, suicide, RNA quality, postmortem delay, age, and brain pH, thus increasing confidence in the validity of their findings.