OBI1 and attention deficit-hyperactivity disorder: Our study explored the potential involvement of lncRNA RNF219‐AS1 in the pathophysiology of ADHD at different phenotypic levels, levels guided by the Research Domain Criteria (RDoC) framework: (i) ADHD caseness and symptom dimension, (ii) executive functions as functional endophenotype, and (iii) potential genetic influence of rs3908461 in RNF219‐AS1 on white matter architecture as brain structural endophenotype.