Targeting took place in ES cells carrying the cre transgene Tg(Prm-cre)70Og, which expresses Cre recombinase under control of the male-germline specific protamine promoter. The insertion of a floxed neo cassette replaced the 3.7 kb coding region. Cre-mediated excision removed the neo cassette upon transmission from male chimeras. (J:98580)