Abstract: Changes in Reelin levels as well as Reelin signaling alter cognitive function. This can be accomplished by administering a therapeutically effective amount of a repeat fragment of Reelin, or a construct formed from fragment repeats of Reelin to a patient or subject. Changes to Reelin levels can be used to treat various neurodegenerative diseases, neuronal insults, or stroke, such as fragile X syndrome, William's syndrome, Rett syndrome, Down's syndrome, Angelman syndrome, autism, ischemia, hypoxia, Alzheimer's disease, and schizophrenia. Reelin can also be used to alter dendritic spine density, diminished long-term potentiation, and diminished synaptic plasticity and associative learning deficits. Constructs formed from repeat region 3 of full length Reelin and repeat region 5 of full length Reelin, or repeat region 3 of full length Reelin and repeat region 6 of full length Reelin have been found particularly useful.