The present invention contemplates isolated polynucleotide for the production of a VHH-containing heavy chain antibody in a mammal and vectors comprising said isolated polynucleotide. Moreover, the invention relates to a transgenic mammal comprising the vector for the production of a VHH-containing heavy chain antibody. Further, the invention relates to VHH-containing heavy chain antibodies as well as methods for the production and cloning of VHH-containing heavy chain antibodies.