Human artificial chromosome vector

A human artificial chromosome (HAC) vector, wherein the HAC comprises:

• a) a gene encoding a human antibody heavy chain, wherein the gene encoding the human antibody heavy chain encodes at least a human IgG (γ) heavy chain, a human IgA (α) heavy chain or both human IgG and IgA heavy chains,
• b) a gene encoding a human antibody light chain, wherein the gene encoding the human antibody light chain encodes either kappa (κ) human antibody light chain or lambda (λ) human antibody light chain, or both κ and λ human antibody light chains, and
• c) a gene encoding a chimeric IgM (μ) heavy chain constant region, wherein the gene encoding the chimeric IgM heavy chain constant region comprises a human IgM heavy chain constant region gene substituted with DNA encoding a bovine or a goat IgM heavy chain constant region gene;
• wherein the gene encoding the chimeric IgM heavy chain constant region encodes constant region (CH) domains CH1, CH2, CH3 and CH4, and transmembrane domains (TM) TM1 and TM2;
• and wherein the encoded human CH4, TM1 and TM2 domains are completely substituted with the encoded CH4, TM1 and TM2 domains of the bovine or the goat IgM heavy chain constant region gene.