GCH1 and attention deficit-hyperactivity disorder: The case reports presented in this article illustrate that a partial BH4 deficiency, as conveyed by the <i>GCH1</i> rs841 variant, may contribute to wider issues in mental and neurological health including depression and ADHD but also severe treatment-resistant anxiety, Premenstrual Dysphoric Disorder, insomnia, complex behavioral issues, and autism.