A 2.3 kb genomic fragment encompassing the 5' DNase I hypersensitive site 3 (HS3) was replaced with a loxP site flanked neomycin resistance gene cassette inserted by homologous recombination. RT-PCR analysis indicated that the presence of neo cassette reduces Hbb expression by an additional 25 to 40% relative to Rr320tm1.1Ley. (J:33053)