The targeting construct for the a-casein gene was generated using a short arm of homology of 679 bp corresponding to nucleotides 208 to 887 downstream of the transcriptional start site (which was isolated as a EcoRV/BssHII fragment from the bacterial artificial chromosome BAC 490H23 of a Research Genetics BeloBAC11 library derived from 129SV mouse DNA) and a long arm of homology of 6696 bp corresponding to nucleotides 1422 to 8117 downstream of the transcriptional start site (isolated as a StuI/EcoRV fragment). The targeting construct carries a hygromycin-thymidine kinase fusion gene under the control of the murine phospho-glycerol kinase (PGK) promoter as selection marker.