One of molecular biology's mightiest tools has been the knockout mouse. Although its name conjures up images of a cartoon animal that boasts, "Here I come to save the day" before duking it out with evil characters, this mouse is no fantasy. The knockout mouse has served as a model for human diseases and shed light on the workings of the brain. Now a new generation of knockout mice developed at MIT packs an even more powerful punch.

Knockout mice are laboratory strains engineered to lack a specific gene. By studying what happens when that gene is missing, researchers gain clues to its function. Until now the clues were fuzzy, says Susumu Tonegawa, director of MIT's Center for Learning and Memory and a Nobel laureate. Researchers could make a mouse lacking a key gene, but it would be missing throughout the mouse's body and its lifetime-from the earliest stages of embryonic development to death. Because a gene may have different functions in different parts of the body or at different times, investigators were hard pressed to draw conclusions from such "first-generation" knockout mice about exactly where and when the missing gene would have otherwise played a role.

To zero in on a gene's function, researchers need to knock it out only in certain locations or during a narrow window of time. That is what Tonegawa, Matthew Wilson-an assistant professor in the departments of Biology and Brain and Cognitive Sciences-and investigators at California Institute of Technology and Columbia University have done. They have created a new kind of knockout mouse in which a specific gene is deleted only in certain cells in the brain and only after neural circuitry has developed. The researchers have worked with a gene thought to affect the development of spatial memory, the mechanism by which we navigate familiar environments.

To create the new strain, the scientists attached an "on-off switch"-a stretch of DNA called a site-specific promoter because it controls activity in particular types of cells-to a molecular device that chops out genes. German researchers had used a similar approach to create a mouse strain lacking a gene in immune-system cells, but applying the technique to the brain was especially tricky, says Tonegawa. His group knew the gene chopper would work in actively dividing cells such as immune cells, but wasn't sure about brain cells, which stop dividing around the time of birth. The researchers also weren't sure that the on-off switch would restrict gene deletion as precisely as they wanted. Although subregions of the brain have quite different functions, the anatomy and organization of brain cells are virtually identical from one subregion to another. The team feared the promoter wouldn't be able to distinguish cells in a specific subregion.