A 1085 bp region containing Tcrd enhancer Edelta was replaced with a loxP site flanked neomycin resistance gene cassette, which was subsequently removed via cre-mediated recombination. A 1 kb PvuII-BamHI genomic fragment containing Tcra enhancer Ealpha (Rr308) was replaced with a sequence containing a loxP site flanked neomycin resistance gene cassette and Tcrd enhancer Edelta (Rr299391). The neo cassette was subsequently deleted through cre-mediated recombination. (J:82395)