An assay for a GCH1 allele and associated genotype for the screening, prediction, diagnosis, prognosis, treatment and treatment response of psychiatric, neuropsychiatric, and neurological disorders, such as schizophrenia, schizoaffective disorder and bipolar disorder, and for defining treatments of such disorders. The presence of a variant in the GCH1 gene, alone or in conjunction with a measurement of low or altered biopterin, or altered BH4 system measures, is used to screen or diagnose subjects at risk for developing a psychiatric, neuropsychiatric, or neurological disorder. The genetic assay, with or without a biopterin or BH4 system assay, may also be used to determine treatment regimens. For subjects with an impaired BH4 system, treatments to increase or normalize biopterin, BH4, or the BH4 system can also be used, such as BH4 supplementation, lithium treatment, phenylalanine treatment, or other treatments and therapies.
