Humanized non-human animals with restricted immunoglobulin heavy chain loci

A method of producing a human immunoglobulin heavy chain variable domain sequence, the method comprising:

• (a) immunizing a genetically modified mouse with an antigen of interest, wherein the mouse comprises in its germline genome:
• (i) an unrearranged human genomic sequence comprising a single human VH gene segment, one or more DH gene segments, and one or more JH gene segments, wherein the single human VH gene segment, the one or more DH gene segments, and the one or more JH gene segments are operably linked to a mouse immunoglobulin heavy chain constant region gene at the endogenous immunoglobulin heavy chain locus, wherein insertion of the unrearranged human genomic sequence disrupts the function of an endogenous ADAM6 protein, and wherein the disruption of the endogenous ADAM6 function is associated with a reduction in fertility in male mice;
• and
• (ii) a nucleic acid sequence that encodes a mouse ADAM6 protein that is functional in a male mouse, wherein the mouse ADAM6 protein is expressed if the mouse is a male mouse such that the male mouse is fertile;
• and
• wherein B cells of the genetically modified mouse express antibodies in response to exposure to an antigen, wherein each antibody includes two immunoglobulin light chains paired with two immunoglobulin heavy chains, wherein each heavy chain comprises a human heavy chain variable domain expressed from a human heavy chain variable region sequence including a VH gene segment that is identical to, or a somatically hypermutated version of, the single human VH gene segment;
• (b) allowing the genetically modified mouse to mount an immune response with respect to the antigen of interest;
• (c) obtaining a B cell from the genetically modified mouse that expresses an antibody that specifically binds the antigen of interest;
• and
• (d) determining an amino acid sequence of a human heavy chain variable domain of the antibody expressed by the B cell, wherein the antibody specifically binds the antigen of interest;
• and
• (e) producing the human heavy chain variable domain.