These findings on serotonin deficiency in preclinical models are supported by human data, as polymorphism in THP1 has been associated with obesity traits—such as waist circumference and BMI—and elevated plasma levels of the serotonin end-product 5-hydroxyindole-3-acetic acid (5-HIAA) have been reported in patients with metabolic syndrome [151,165]. Here, GLI2 is linked to metabolic syndrome.