CONTEXT: Convergent data make 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) a candidate gene for schizophrenia. Reduced expression has been reported in the schizophrenic brain. The CNP gene maps to a region to which we have reported linkage to schizophrenia. Mice in which the CNP gene has been knocked out display central nervous system pathological characteristics reminiscent of some features observed in schizophrenia. 2',3'-Cyclic nucleotide 3'-phosphodiesterase is used as a marker of myelin-forming cells and is detectable in cells of oligodendrocyte lineage throughout life. Because CNP is thought to be important for oligodendrocyte function, altered expression has been interpreted as supportive of the hypothesis that altered oligodendrocyte function may be an etiological factor in schizophrenia. However, it is unclear whether the observed changes in the schizophrenic brain are primary or secondary. OBJECTIVES: To determine if CNP expression is influenced by DNA polymorphisms and to verify if these polymorphisms are associated with schizophrenia. DESIGN: Allele-specific messenger RNA expression assay and genetic association studies. SETTING: Unrelated subjects were ascertained from secondary psychiatric inpatient and outpatient services. PARTICIPANTS: We used brain tissue from 60 anonymous individuals with no known psychiatric disorder; a case-control sample of 708 white individuals from the United Kingdom meeting DSM-IV criteria for schizophrenia matched for age, sex, and ethnicity to 711 blood donor controls; and a pedigree with DNA from 6 affected siblings and 1 parent, showing evidence for linkage to CNP. MAIN OUTCOME MEASURES: Association between allele and gene expression. Association between allele and schizophrenia. RESULTS: The exonic single nucleotide polymorphism rs2070106 was associated with CNP expression (P<.001). Compatible with underexpression of CNP messenger RNA in schizophrenia, the lower-expressing A allele was significantly associated with schizophrenia (P = .04) in the case-control sample. All affected individuals in the linked pedigree were homozygous for the lower-expression allele, providing independent support for the association (P = .03). CONCLUSIONS: Our data support the hypothesis that reduced CNP expression in the schizophrenic brain is relevant to disease etiology and therefore provide support for the general hypothesis that altered oligodendrocyte function is an etiological factor in schizophrenia.