A method of preparing an antibody, the method comprising: (a) immunising a fertile rodent whichy is a mouse or rat with a target antigen using a prime and boost protocol or a Repetitive IMmunisation Multiple Sites (RIMMS) protocol such that the mouse or rat produces antibodies; and (b) isolating nucleotide sequences encoding at least the heavy and the light chain variable regions of such an antibody; and subsequently joining the nucleotide sequence encoding the heavy chain variable region of said antibody to a nucleotide sequence encoding a human constant region, and joining the nucleotide sequence encoding the light chain variable region of said antibody to a nucleotide sequence encoding a human constant region, wherein the fertile mouse or rat is optionally male, and is homozygous for a transgenic antibody heavy chain locus, the mouse or rat having a genome that (i) comprises each transgenic heavy chain locus on a respective copy of a first chromosome, wherein said first chromosome is chromosome 12 when the rodent is a mouse, or is chromosome 6 when the rodent is a rat; and (ii) is inactivated for endogenous antibody heavy chain expression; wherein each first chromosome of the genome comprises (iii) a transgenic antibody heavy chain locus comprising one or more human VH gene segments, one or more human D gene segments and one or more human JH gene segments operably connected upstream of the respective mouse or rat heavy chain constant region, optionally Cmu and/or Cgamma; (iv) a deletion of all or part of the endogenous heavy chain VDJ region of said chromosome to inactivate endogenous antibody heavy chain expression, wherein the deletion includes ADAM6; wherein the genome comprises (v) an insertion of one or more expressible ADAM6 genes; wherein the genome comprises inserted expressible mouse ADAM6a and ADAM6b genes; or wherein the genome comprises an inserted expressible rat ADAM6 gene, wherein each ADAM6 nucleic acid sequence is inserted so that it is operably connected to a promoter for expression and each ADAM6 nucleic acid sequence is inserted within one or both transgenic heavy chain loci. (a) immunising a fertile rodent whichy is a mouse or rat with a target antigen using a prime and boost protocol or a Repetitive IMmunisation Multiple Sites (RIMMS) protocol such that the mouse or rat produces antibodies; and (b) isolating nucleotide sequences encoding at least the heavy and the light chain variable regions of such an antibody; and subsequently joining the nucleotide sequence encoding the heavy chain variable region of said antibody to a nucleotide sequence encoding a human constant region, and joining the nucleotide sequence encoding the light chain variable region of said antibody to a nucleotide sequence encoding a human constant region, wherein the fertile mouse or rat is optionally male, and is homozygous for a transgenic antibody heavy chain locus, the mouse or rat having a genome that (i) comprises each transgenic heavy chain locus on a respective copy of a first chromosome, wherein said first chromosome is chromosome 12 when the rodent is a mouse, or is chromosome 6 when the rodent is a rat; and (ii) is inactivated for endogenous antibody heavy chain expression; wherein each first chromosome of the genome comprises (iii) a transgenic antibody heavy chain locus comprising one or more human VH gene segments, one or more human D gene segments and one or more human JH gene segments operably connected upstream of the respective mouse or rat heavy chain constant region, optionally Cmu and/or Cgamma; (iv) a deletion of all or part of the endogenous heavy chain VDJ region of said chromosome to inactivate endogenous antibody heavy chain expression, wherein the deletion includes ADAM6; wherein the genome comprises (v) an insertion of one or more expressible ADAM6 genes; wherein the genome comprises inserted expressible mouse ADAM6a and ADAM6b genes; or wherein the genome comprises an inserted expressible rat ADAM6 gene, wherein each ADAM6 nucleic acid sequence is inserted so that it is operably connected to a promoter for expression and each ADAM6 nucleic acid sequence is inserted within one or both transgenic heavy chain loci. (i) comprises each transgenic heavy chain locus on a respective copy of a first chromosome, wherein said first chromosome is chromosome 12 when the rodent is a mouse, or is chromosome 6 when the rodent is a rat; and (ii) is inactivated for endogenous antibody heavy chain expression; wherein each first chromosome of the genome comprises (iii) a transgenic antibody heavy chain locus comprising one or more human VH gene segments, one or more human D gene segments and one or more human JH gene segments operably connected upstream of the respective mouse or rat heavy chain constant region, optionally Cmu and/or Cgamma; (iv) a deletion of all or part of the endogenous heavy chain VDJ region of said chromosome to inactivate endogenous antibody heavy chain expression, wherein the deletion includes ADAM6; wherein the genome comprises (v) an insertion of one or more expressible ADAM6 genes; wherein the genome comprises inserted expressible mouse ADAM6a and ADAM6b genes; or wherein the genome comprises an inserted expressible rat ADAM6 gene, wherein each ADAM6 nucleic acid sequence is inserted so that it is operably connected to a promoter for expression and each ADAM6 nucleic acid sequence is inserted within one or both transgenic heavy chain loci.