Enables GTP cyclohydrolase I activity. Involved in positive regulation of heart rate. Acts upstream of or within regulation of lung blood pressure. Predicted to be located in cytoplasmic vesicle; cytosol; and nucleus. Predicted to be part of protein-containing complex. Predicted to be active in cytoplasm. Is expressed in several structures, including alimentary system; genitourinary system; nervous system; respiratory system; and sensory organ. Used to study BH4-deficient hyperphenylalaninemia B. Human ortholog(s) of this gene implicated in BH4-deficient hyperphenylalaninemia B; bipolar disorder; dystonia; and dystonia 5. Orthologous to human GCH1 (GTP cyclohydrolase 1). [provided by Alliance of Genome Resources, Apr 2022]