Hyperphenylalaninemia, Bh4-Deficient, B, also known as gtp cyclohydrolase i deficiency, is related to hyperphenylalaninemia, bh4-deficient, a and tetrahydrobiopterin deficiency, and has symptoms including lethargy, muscle rigidity and seizures. An important gene associated with Hyperphenylalaninemia, Bh4-Deficient, B is GCH1 (GTP Cyclohydrolase 1), and among its related pathways/superpathways are Metabolism and Metabolism of cofactors. Affiliated tissues include eye and endothelial, and related phenotypes are tremor and global developmental delay