CYP2D6 and Anxiety: The higher abundance of 5-HT and 5-hydroxyindole acetic acid in brain was found in human CYP2D6-transgenic mice than wild-type mice and the transgenic mice were more able to adapt to anxiety (Cheng et al., 2013), providing direct evidence that brain CYP2D6 catalyzes 5-HT formation from 5-MT (Figure 1B).