ES cells containing Tcrbtm1Slek, which contain a loxP site in place of the Dbeta2-Jbeta2 gene cluster, were additionally targeted to replace the 5' Dbeta1 12-RSS with the Jbeta1.2 12-RSS. The allele also contains loxP and HindIII sites between Dbeta1 and Jbeta1.1, as well as the conversion of a bamHI site 3' of Jbeta1.2 to a PstI site. A floxed neo cassette was removed from the locus. (J:119552)