BL21 is RecA+, whereas DH5alpha is RecA- and thus more appropriate for cloning purposes. BL21 also produces lower levels of proteases and thus better for expression of recombinant proteins. -Emir "fisher" <foolio_j88 at yahoo.com> wrote in message news:%KW5a.1377$XR3.40892 at news.itd.umich.edu... > What's the difference between these two?? > >