Here, we sought to determine whether Tcf4 haploinsufficiency, which has been associated with autism and intellectual disability in humans, affects adult hippocampal neurogenesis. Analysis of Tcf4 heterozygote knockout mice revealed that Tcf4 haploinsufficiency is associated with a smaller size of the hippocampal neural stem/progenitor cell pool and impaired maturation and survival of adult-born dentate granule neurons. The gene discussed is TCF4; the disease is autism.