New neurons in the adult brain transiently express molecules related to neuronal development, such as the polysialylated form of neural cell adhesion molecule, or doublecortin (DCX). These molecules are also expressed by a cell population in the rat paleocortex layer II, whose origin, phenotype, and function are not clearly understood. We have classified most of these cells as a new cell type termed tangled cell. Some cells with the morphology of semilunar–pyramidal transitional neurons were also found among this population, as well as some scarce cells resembling semilunar, pyramidal. and fusiform neurons (Nacher et al., 2002). We have found that none of these cells in layer II express markers of glial cells, mature, inhibitory, or principal neurons. They appear to be in a prolonged immature state, confirmed by the coexpression of DCX, TOAD/Ulip/CRMP-4, A3 subunit of the cyclic nucleotide-gated channel, or phosphorylated cyclic adenosine monophosphate response element–binding pro- tein. Moreover, most of them lack synaptic contacts, are covered by astroglial lamellae, and fail to express cellular activity markers, such as c-Fos or Arc, and N-methyl-d-aspartate or glucocorticoid receptors. We have found that none of these cells appear to be generated during adulthood or early youth and that most of them have been generated during embryonic development, mainly in E15.5 (Gomez-Climet et al., 2008).  Recent studies in our lab have demonstrated that t olfactory bulbectomy, but not odor conditioned aversion, induces the differentiation of immature neurons in the adult rat piriform cortex (Gomez-Climent et al., 2011). These  immature neurons are also present, although with a wider distribution, in the cerebral cortex layer II of adult cats (Varea et al., 2011). A detailed review on the current knowledge of these immature neurons can be found in Gomez-Climent et al. (2010).