In this project we propose to generate a bovine herd with a disrupted prion protein (PrP) gene by using homologous recombination on somatic cells (bovine ES cells are not yet available) and somatic cloning. Following the first round of homologous recombination for the inactivation of one allele of the PrP gene in both male and female cells lines, the modified cells will be used for embryo production by somatic cloning. The embryos will be transferred in recipient heifers and the derived heterozygous animals will be bred, at due time, to obtain the homozygous offspring. In parallel, the second round of homologous recombination will be attempted on embryonic cell lines derived from the cloned heterozygous embryos. The homozygous animals, with both alleles of the PrP gene inactivated, will be founders of a nucleus bovine herd strictly BSE free. Once it will be demonstrated that such animals are resistant to experimental prion infection the products of such herd, namely those of biomedical interest, will be totally devoid of any risk of BSE transmission.