The present invention provides in a first aspect a mouse in which the λ (lambda) light chain locus has been functionally silenced. In one embodiment, the mouse λ light chain locus was functional silenced by deletion of gene segments coding for the λ light chain locus. In a further aspect, a mouse containing functionally silenced λ and κ (kappa) L chain loci was produced. The invention is useful for the production of antibodies, for example heterologous antibodies, including heavy chain only antibodies.