Since its first application in mice almost 10 yr ago, the Cre/loxP has become the system of choice to study gene function in vivo in a cell-type, stage-specific, and inducible manner. This chapter provides a set of updated protocols that will help the reader to construct a vector for conditional gene targeting, to in vitro manipulate embryonic stem (ES) cells and to rapidly identify successfully targeted ES colonies. It also provides an updated list of Cre strains currently used to assess gene function at defined stages of B-cell development and guidelines to generate single-copy, knock-in transgenes regulated in a Cre-dependent manner.
