CTCF target site 4 within the imprinting control region (ICR) was modified from a central GTGG motif to ATAT via homologous recombination. A floxed neomycin resistance gene cassette that was included in the targeting vector was excised via in vivo cre-mediated recombination in the germ line. Southern blot analysis and sequence analysis confirmed recombination. (J:89915)