In this study, Ms. Tyrer examined the role of the brain protein, serotonin transporter (5-HTT), and its role in Seasonal Affective Disorder (SAD). 5-HTT is a brain protein that clears serotonin from synapses, and serotonin depletion has been associated with low mood. Ms. Tyrer utilized a type of positron emission tomography (PET) imaging, the [11C]DASB brain scanning method, in order to measure changes between 5-HTT levels in patients with SAD, compared to healthy controls, in mood-regulating regions of the brain. She was able to find a significant positive correlation with seasonal variation and changes in 5-HTT levels in the prefrontal cortex and anterior cingulate cortex regions of the brain, between patients with SAD compared to healthy controls. This finding is the first to report a brain biomarker to be associated with SAD, and has important implications in optimizing prevention and/or treatment strategies in SAD.