In the more general context of fatigue (i.e. notrestricted to multiple sclerosis), positron emission tomography (PET) studies havedemonstrated altered 5-HT transporter distribution in patients with chronic fatiguesyndrome as compared to controls, as well as in patients with Parkinson’sdisease complaining of fatigue as compared to those without fatigue.17 Here, SLC6A4 is linked to multiple sclerosis.