The genomes of both mice and humans consist of about 35,000 to 100,000 different genes, not accounting for possible splice variants. With this seemingly limited repertoire of molecular components, both mice and human are able to develop a functioning nervous system capable of the vast array of mental abilities with which we are all familiar. Our knowledge of the molecular mechanisms by which the mammalian brain develops and functions in the adult has progressed rapidly in the past decade in part due to technological advances in our availability to manipulate the mouse genetically. These methods are referred to as reverse genetic approaches. 44In contrast to classical forward genetics where one starts with or screens for a phenotypic variant and then tracks down the genetic variation that produces it, in reverse genetics we begin with a gene of interest and generate animals, mice in this case, in which the gene is altered. The mouse is then examined for the phenotypic consequences, if any, of the genetic manipulation.