Chances are if you use mice in your research, you are intimately acquainted with the Cre/LoxP recombination system. (For the uninitiated out there, this is a system to achieve tissue- or cell type-specific expression or deletion of a given gene through the use of selectively expressed Cre recombinase and strategically placed recombination sites.) The Cre Portal of MGI is a nice tool to find the perfect Cre lines for your research. Visitors to the site can search for Cre alleles driven by a particular promoter, or they can search for alleles that are known to be expressed in a particular tissue/anatomical system (because we all know that the expected and actual expression patterns are not necessarily the same). The database includes approximately 500 Cre recombinase-containing knock-in alleles, as well as approximately 1,300 recombinase-containing transgenes.