Reelin (or Reln) is a large glycoprotein that is secreted by Cajal-Retzius cells in the forebrain and by granule neurons in the cerebellum. Reelin was shown to be mutated in “reeler” mice, a mutation that is associated with widespread disruption of laminated regions of the brain, leading to impaired motor coordination, tremors and ataxia. Reelin protein expression is complex and changes throughout development. Reelin appears to function upstream of Dab1 in a signaling pathway that controls cell positioning in the developing brain and is also thought to be a direct effector of the neurotrophin BDNF.