L2HGDH and attention deficit-hyperactivity disorder: Some illustrative examples of recent discoveries reported after Fernandez-Castillo et al. conducted their systematic review include patients presenting with ADHD symptoms due to deficiency of L-2-hydroxyglutarate dehydrogenase (L2HGDH) (34), X-linked creatine transporter deficiency (35) and HUWE1 mutations (36).