NeuN (Fox-3, Rbfox3, or Hexaribonucleotide Binding Protein-3), a protein which is a homologue to the protein product of a sex-determining gene in Caenorhabditis elegans, is a neuronal nuclear antigen that is commonly used as a biomarker for neurons. NeuN antibodies are widely used to label neurons. A few neuronal cell types are not recognized by NeuN antibodies, such as Purkinje cells, stellate and Golgi cells of the cerebellum, olfactory Mitral cells, retinal photoreceptors and spinal cord gamma motor neurons. However the vast majority of neurons are strongly NeuN positive, and NeuN immunoreactivity has been widely used to identify neurons in tissue culture and in sections and to measure the neuron/glia ratio in brain regions. NeuN immunoreactivity becomes obvious as neurons mature, typically after they have downregulated expression of Doublecortin, a marker seen in the earliest stages of neuronal development.