Genetic variations, transcriptional factors, promoter DNA methylation, tobacco smoking-induced alterations in levels of CHRNA7 mRNA and protein expression, α7 subunit trafficking, folding, and assembly into homopentameric receptors, as well as the dysfunction of cell-surface α7nAChRs, are associated with AD, PD, schizophrenia, and depression. Here, CHRNA7 is linked to major depressive disorder.