A targeting vector inserted into the hypermutation site 43 bp downstream of segment 4 a 93 bp oligonucleotide, consisted of 10 repeats of a 9-mer sequence, TATTATTAA, with 4 repeats flanking 3 Gs separated by the same 9-mer sequence (oligo-G). Cre-mediated recombination removed the floxed neomycin resistance cassette. (J:221629)