The present invention describes transgenic animals with human(ized) immunoglobulin loci and transgenes encoding human(ized) Ig.alpha. and/or Ig.beta. sequences. Of particular interest are animals with transgenic heavy and light chain immunoglobulin loci capable of producing a diversified human(ized) antibody repertoire that have their endogenous production of Ig and/or endogenous Ig.alpha. and/or Ig.beta. sequences suppressed. Simultaneous expression of human(ized) immunoglobulin and human(ized) Ig.alpha. and/or Ig.beta. results in normal B-cell development, affinity maturation and efficient expression of human(ized) antibodies.
