In addition, genome-wide association studies have found associations between CDK5 and numerous psychiatric phenotypes of interest, including schizophrenia, depression, attention-deficit hyperactivity disorder, sleep difficulties, chronotype, and smoking cessation (40), whereas previous epigenome-wide associations studies have identified associations between CDK5 and schizophrenia, smoking, and maternal smoking (41). This evidence concerns the gene CDK5 and major depressive disorder.