GCH1 and Parkinson disease: Since a single nucleotide polymorphism (SNP) at the GCH1 gene encoding the rate-limiting enzyme for BH4 synthesis was associated with PD in a large-scale genome-wide association study (GWAS) meta-analysis (32), sepiapterin treatment might be more effective in PD with the SNP of GCH1 than without the SNP of GCH1.