The Ca(2+)-binding proteins (CBPs) parvalbumin, calbindin, and calretinin are phenotypic markers of terminally differentiated neurons in the adult brain. Although subtle phylogenetic variations in the neuronal distribution of these CBPs may occur, morphologically and functionally diverse subclasses of interneurons harbor these proteins in olfactory and corticolimbic areas. Secretagogin (scgn) is a recently cloned CBP from pancreatic beta and neuroendocrine cells. We hypothesized that scgn is expressed in the mammalian brain. We find that scgn is a marker of neuroblasts commuting in the rostral migratory stream. Terminally differentiated neurons in the olfactory bulb retain scgn expression, with scgn being present in periglomerular cells and granular layer interneurons. In the corticolimbic system, scgn identifies granule cells distributed along the dentate gyrus, indusium griseum, and anterior hippocampal continuation emphasizing the shared developmental origins, and cytoarchitectural and functional similarities of these neurons. We also uncover unexpected phylogenetic differences in scgn expression, since this CBP is restricted to primate cholinergic basal forebrain neurons. Overall, we characterize scgn as a neuron-specific CBP whose distribution identifies neuronal subtypes and hierarchical organizing principles in the mammalian brain.