The invention provides transgenic animals comprising some or all components of human heavy and / or light chain immunoglobulin variable region loci, methods of making such animals, methods of making human antibodies using such animals, and methods of treatment using human antibodies prepared in such animals, wherein the animal comprises in its genome a plurality of human heavy chain V gene segments and / or a plurality of human light chain V gene segments, or both, all of the plurality of human heavy chain V gene segments immediately following a same first leader peptide coding sequence and all of the plurality of human light chain V gene segments immediately following a same second leader peptide coding sequence. The invention also provides a polynucleotide construct comprising two or more human heavy chain or light chain precursor / V gene segments comprising the same leader peptide coding sequence. Such animals, constructs, and methods can be used to efficiently produce an optimal diverse population of antibodies against an antigen of interest, such as a therapeutic antigen of interest.