Using site-directed mutagenesis, the 0.9 kb intervening sequence (IVS) between the CTCF sites was deleted. A floxed neomycin resistance gene cassette inserted downstream of the modified sequence was removed by cre-mediated recombination. The size and CpG content of the imprinting control region (ICR) are decreased 50% and 35%, respectively. (J:173610)