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Adequate maturation and integration of the adult-generated neurons into the circuit of the hippocampus would be crucial for normal cognitive functions and emotional behaviors.Disruption of the process could result in some disturbance in mental health.Previously, we reported that mice heterozygous for a null mutation of alpha-CaMKⅡ, a key molecule in synaptic plasticity, have profoundly dysregulated behaviors including hyper-locomotor activity and a severe working memory deficit, which are endophenotypes of schizophrenia and other psychiatric disorders.Surprisingly, we found that almost all the neurons in the dentate gyrus (DG) of the mutant mice failed to mature at molecular, morphological and electrophysiological levels.Upon these findings, we have been trying to find the strains of mutant mice that have "immature dentate gyrus (iDG)" among approximately 120 strains of genetically engineered mice with which we have conducted our comprehensive behavioral test battery.So far, we have identified four other strains of mutant mice that have a phenotype strikingly similar to iDG, including the forebrain specific calcineurin knockout mice and the mice lacking a transcription factor, schnurri-2.We also found that chronic fluoxetine treatment can induce "dematuration" resulting in iDG-like phenotype in wild type mice.Gene and protein expression patterns in the DG of these mice are similar to those found in the post-mortem brains of psychiatric patients, such as schizophrenia and bipolar disorder.Interestingly, a bioinformatics analysis revealed that the transcriptome pattern of the hippocampus of the mice with iDG-like phenotype had highly significant similarity to those obtained from the experiments involving injuries, infections, wounds, aging, and sleep deprivation, suggesting that iDG is associated with a phenomenon similar to inflammation.Based on these results, we propose that an iDG in adulthood might serve as a promising candidate endophenotype of schizophrenia and its related psychiatric disorders.