Targeted insertion of thymidine kinase into the C57BL/6J derived Chromosome 5 Htt locus in the v6.4 ES cell line was followed by 400 Gy of gamma radiation, FIAU selection and injection into C67BL/6J blastocysts. This deletion includes the full Wolf-Hirschhorn syndrome critical region but not D5Mit 149 or D5Mit388. (J:67077)