SLC6A4 and attention deficit-hyperactivity disorder: Two studies used a candidate gene approach and both reported positive associations, but between different MRI features and different genes: the first between DNAm at the SLC6A4 promoter and cortical thickness in an uncontrolled case series of 102 children with ADHD (Park et al., 2015); and the second between DNAm at FKBP5 introns and dlPFC activation during the go no-go task in a case series of young adults with a history of early life stress exposure (Harms et al., 2017).