Analysis of genetically engineered mice is crucial for our understanding of the in vivo function of genes and proteins in the whole organism. This includes inactivation of a gene or the generation of specific mutations. The development of knockout and transgenic technologies in the mouse, therefore, represents a powerful tool for elucidating gene function, for modeling of human diseases, and potentially for the evaluation of drugs. In particular, conditional gene targeting applying the Cre/loxP-mediated recombination system is increasingly used to evaluate the role of the gene of interest in a cell-type-specific or even inducible manner. The experimental steps start with the characterization of the gene locus, followed by construction of a vector, gene targeting in ES cells, and establishment of mouse lines carrying the desired mutation. These are then bred to transgenic mice expressing Cre recombinase in a tissue-specific manner, thus allowing gene inactivation in a cell type of interest.