Using the first 377 amino acids of neuronal nitric oxide synthase (NOS1) as bait in a yeast 2-hybrid screen, Jaffrey et al. (1998) isolated cDNAs encoding NOS1AP, which they called CAPON. CAPON appears to have 2 alternate splice forms, one encoding a 125-amino acid protein and the other encoding a 327-amino acid protein. The 125-amino acid C-terminal fragment of CAPON specifically interacted with NOS1 in a 2-hybrid system. The full-length cDNA produces a 503-amino acid protein. CAPON displayed no significant homology to any other class of protein except for an N-terminal 145-amino acid stretch that has residues suggestive of a phosphotyrosine-binding (PTB) domain. Northern blot analysis detected a predominant 7.5-kb transcript only in brain regions, with no expression evident in adrenal, bladder, heart, kidney, lung, and skeletal muscle.