A targeting vector was designed to mutate the KI-KII element (Rr163) such that 8 and 9 bp were replaced, respectively. The sequence of KI was mutated from TCCACGCATGCTTGGA to TGTACGCTGCTGATGA and KII was mutated from TCCTCTTCAGTGAGGAG to AGCTCTTCTAGACTGA. LoxP sites were inserted to surround 4 kb encompassing the promoter of the long J-C K germline transcript, located immediately upstream of the KI-KII element. This sequence was deleted through subsequent cre-mediated recombination. (J:110775)