NOS1 (nitric oxide synthase 1) is an enzyme that catalyzes nitric oxide from L-arginine. Nitric oxide itself functions as a diffuse messenger with neurotransmitter properties in the brain and is thought to play a role in long term potentiation, stroke-associated neurotoxicity, smooth muscle neural regulation, and neurodegenerative disease. A distinct isoform of NOS1 found in macrophages is responsible for regulating bactericidal and tumoricidal functions. NOS1 is involved in neurodegenerative and neuropsychiatric diseases including bipolar disorder, schizophrenia and Alzheimer’s disease. In immunohistochemistry, NOS1 has highest cytoplasmic positivity in the cerebellum, cerebral cortex and hippocampus of the brain, and also has moderate positivity in skeletal muscle. References: Am. J. Med. Genet. B Neuropsychiatr. Genet. 124B (1): 73–5, PMID: 14681919; Mol. Psychiatry. 11 (3): 286–300, PMID: 16389274; Neuroscience. 1998 Oct;86(3):695-9, PMID: 9692709; Oxidative Medicine and Cellular Longevity. 2016. 3806157, DOI: 10.1155/2016/3806157;