Fox3 is one of a family of 3 mammalian Fox homologs. Fox was discovered in C. elegans as a gene involved in sex determination, and the name Fox is an acronym of "Feminizing locus on X". The Fox protein and its 3 mammalian homologs are all about 46kD proteins each of which includes a central highly conserved RRM type RNA recognition motif, which corresponds to a small ~70 amino acid structure consisting of 4 beta strands and two alpha-helices. An alternate name for Fox 3 is hexaribonucleotide binding protein 3, and these proteins are believed to have a role in the regulation of mRNA splicing. Much interest has focused on Fox3 as a result of the recent finding that this protein corresponds to NeuN, a neuronal nuclear antigen. NeuN was first described in 1994 by Mullen et al. (2), who raised a series of monoclonal antibodies to mouse antigens with the original intent of finding mouse species specific markers which would be useful in transplantation experiments. In the event they obtained a hybridoma clone, called mAb A60, which proved to bind an antigen expressed only in neuronal nuclei, and which appeared to work on all vertebrates. A few neuronal cell types were not recognized by the the NeuN antibody, such as Purkinje cells in the cerebellum and photoreceptors in the retina, but the vast majority of neurons were, making this antibody very useful for the identification of neurons. The protein bound by this antibody was not characterized, though the molecular weight of this protein was shown to be three closesly spaced bands running at 46-48kD on SDS-PAGE gels. The exact identity of the NeuN protein was not elucidated or, despite several attempts, for may years later. Despite this the mAb A66 antibody has become very widely used as a robust marker of neurons and neuronal stem cells. Recently Kim et al. showed that NeuN corresponds to Fox3 (3). Fox3 is therefore a protein which has a funciton in RNA splicing and is expressed heavily and specifically in neuronal nuclei. Our antibody was raised against the N-terminal 100 amino acids of human Fox3 as expressed in and purified from E. coli. We did not use full length Fox3 as immunogen since the three mammalian Fox1 homologs, namely Fox1, Fox2 and Fox3, include virtually identical RRM motifs. The N-terminal region of the three molecules are much more variable in the three molecules so antibodies specific for each of the three molecules can therefore be generated. The HGNC name for this protein is RBFOX3.