Gene targeting in embryonic stem (ES) cells coupled with the site-specific Cre/lox P recombination system offers unique opportunities to identify and analyze the roles of cis -acting sequences in the regulation of imprinted gene expression. Although several different approaches have been described to engineer large chromosomal rearrangements in ES cells, these strategies can be labor-intensive and often require several subcloning of the original stem cells, therefore limiting the chances of obtaining germ line transmission of the mutation introduced. Here we describe an alternative approach which is based on in vivo recombination, therefore limiting the number of steps performed in ES cells and allowing to take advantage of the growing number of lox P insertional mutations already available in transgenic mice.