Neurogenic evangelism: comment on Urbach et al. ( 2013). 
The continued production of new dentate granule cells throughout life has led to the idea that neurogenesis critically modulates hippocampal memory function.  Consistent with this view, many studies have shown that experimental reduction of adult neurogenesis impairs hippocampal memory formation.  However, impairments are not universally observed following suppression of adult neurogenesis.  A new article by Urbach et al.  falls into this latter category.  A global deletion of the cyclin D2--a cell cycle regulation gene--induces a profound reduction in adult neurogenesis.  Yet, despite this reduction, this knockout mouse was able to acquire a hippocampus-dependent spatial memory normally.  These results suggest that ongoing adult neurogenesis is not necessary for many forms of hippocampus-dependent learning, and in this commentary, I discuss the implications of these types of results for the field.