GCHFR (GTP cyclohydrolase I feedback regulator) encodes the regulatory protein GFRP, which mediates tetrahydrobiopterin (BH4) feedback inhibition of GTP cyclohydrolase I (GCH1), the rate-limiting enzyme in BH4 biosynthesis 1. This inhibition is reversed by L-phenylalanine, enabling metabolic sensing of cellular cofactor availability 2. GFRP binds GCH1 to form inhibited complexes when BH4 levels are high, but rapidly dissociates in the presence of phenylalanine, promoting BH4 synthesis 2. Since BH4 is essential for synthesis of neurotransmitters including serotonin, dopamine, and nitric oxide, GCHFR variants have psychiatric implications. Promoter polymorphisms (rs7164342, rs7163862) affect transcriptional activity and may influence SSRI antidepressant response, with the TT haplotype showing reduced transcription and altered antidepressant efficacy 3. GCHFR dysregulation has been identified in multiple diseases: altered expression in asthma pathogenesis 4, differential regulation in common variable immunodeficiency with duodenal inflammation 5, schizophrenia (associated with systemic BH4 deficiency) 6, and gout (with causal relationships to specific gut microbial taxa) 7. These findings suggest GCHFR represents a therapeutic target across metabolic, immunological, and neuropsychiatric disorders.