In the past several years, a multitude of genes has been linked to mental retardations in humans, in particular
the most commonly occurring X-linked mental retardations. An emerging idea is that structural and functional abnormalities
in spines and synapses are a common feature of mental retardations. Consistent with this concept, a number of genes
that control synaptic function have been implicated in X-linked mental retardations. In this review we discuss aberrations
in spine and synapse structure and function in X-linked human mental retardations and corresponding mouse models with
mutations in genes that modulate synaptic structure and function. A thorough understanding of these synaptic structures
and abnormalities would enhance our knowledge of the normal process of learning and understand how synaptic aberrations
contribute to cognitive deficits such as those observed in mental retardations.