Isoform 1 is found in neonatal brain, and in a wide variety of adult non-neuronal tissues. Isoform 2 is expressed predominantly in the neurons, but is also found in all other tissues at much lower levels. Isoform 1 and isoform 2 are detected in the lung and heart. Isoform 1 is expressed at higher levels than isoform 2 in the testis and liver and both isoforms are not detected in the skeletal muscle. Isoform 3 with the 16-amino-acid insert is only found in the brain while isoform 3 without the 16-amino-acid insert is found in the lung.