Controlling genetic background during the construction and testing of mutants is complex. Here, we propose that the genetic background of the mutants should always be described in detail, and that any background used should be easily recreated from available stocks. We also propose that both transgenic and gene-targeted mice be generated and maintained in inbred genetic backgrounds (i.e., either 129/J and/or C57BL/6). We propose the study of F1 hybride mice whenever possible (50% C57BL/6 and 50% 129/J). It is important to standardize the genetic background of the mutants studied the facilitate the comparison of results between experiments and among laboratories.