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