In central nervous system structures, the glucocorticoid receptor is gaining interest as a novel representative of neuroendocrine integration, functioning as a major component of endocrine influence - specifically the stress response - upon the brain. The receptor is now implicated in both short and long-term adaptations seen in response to stressors and may be critical to the understanding of psychological disorders, including some or all subtypes of depression and post-traumatic stress disorder ( PTSD ).  Indeed, long-standing observations such as the mood dysregulations typical of Cushing's disease demonstrate the role of corticosteroids in regulating psychologic state; recent advances have demonstrated interactions with norepinephrine and serotonin at the neural level.  
The predominant enzyme responsible for metabolism of tacrolimus is CYP3A5 . Genetic variations within CYP3A5 that result in changes to the activity of the CYP3A5 protein can affect concentrations of tacrolimus within the body. In particular, individuals who are homozygous for the G allele at the single nucleotide polymorphism (SNP) rs776746 (also known as CYP3A5 *3/*3) have a non-functional CYP3A5 protein. The frequency of the G allele varies worldwide, from 4% in some African populations to 80–90% in Caucasian populations.  Across a large number of studies, individuals homozygous for the G allele have been shown to have higher concentrations of tacrolimus and require lower doses of the drug, as compared to individuals who are not homozygous for the G allele. Achieving target concentrations of tacrolimus is important – if levels are too low, then there is a risk of transplant rejection , if levels are too high, there is a risk of drug toxicities. There is evidence to suggest that dosing patients based on rs776746 genotype can result in faster and more frequent achievement of target tacrolimus levels. However, there is a lack of consistent evidence as to whether dosing based on rs776746 genotype results in improved clinical outcomes (such as a decreased risk for transplant rejection or drug toxicities), likely because patients taking tacrolimus are subject to therapeutic drug monitoring .