A method of replacing in situ, in part, in a mouse embryonic stem cell, an endogenous immunoglobulin heavy chain variable region gene locus with an orthologous human gene locus, to create a modified immunoglobulin locus that produces hybrid antibodies containing human variable regions and mouse constant regions, said method comprising: (a) obtaining a large cloned genomic fragment greater than 20kb containing a first part of the orthologous human gene locus; (b) using bacterial homologous recombination to genetically modify the cloned genomic fragment of (a) to create a first targeting vector for use in the ES cell; (c) introducing the first targeting vector of (b) into the ES cell to replace in situ, in part, the endogenous immunoglobulin heavy chain variable region gene locus; (d) using a quantitative assay to detect modification of allele in the ES cell of (c) to identify an ES cell in which the endogenous heavy chain immunoglobulin variable region gene locus has been replaced in situ, in part, with the orthologous human gene locus; (e) obtaining a large cloned genomic fragment greater than 20kb containing a second part of the orthologous human gene locus that differs from the fragment of (a); (f) using bacterial homologous recombination to genetically modify the cloned genomic fragment of (e) to create a second targeting vector for use in the ES cell; (g) introducing the second targeting vector of (f) into the ES cell identified in step (d) to replace in situ, in part, the endogenous immunoglobulin heavy chain variable region gene locus; and (h) using a quantitative assay to detect MOA in the mouse ES cell of (g) to identify an ES cell in which the endogenous immunoglobulin heavy chain variable region gene locus has been replaced in situ, in part, with the orthologous human gene locus. (a) obtaining a large cloned genomic fragment greater than 20kb containing a first part of the orthologous human gene locus; (b) using bacterial homologous recombination to genetically modify the cloned genomic fragment of (a) to create a first targeting vector for use in the ES cell; (c) introducing the first targeting vector of (b) into the ES cell to replace in situ, in part, the endogenous immunoglobulin heavy chain variable region gene locus; (d) using a quantitative assay to detect modification of allele in the ES cell of (c) to identify an ES cell in which the endogenous heavy chain immunoglobulin variable region gene locus has been replaced in situ, in part, with the orthologous human gene locus; (e) obtaining a large cloned genomic fragment greater than 20kb containing a second part of the orthologous human gene locus that differs from the fragment of (a); (f) using bacterial homologous recombination to genetically modify the cloned genomic fragment of (e) to create a second targeting vector for use in the ES cell; (g) introducing the second targeting vector of (f) into the ES cell identified in step (d) to replace in situ, in part, the endogenous immunoglobulin heavy chain variable region gene locus; and (h) using a quantitative assay to detect MOA in the mouse ES cell of (g) to identify an ES cell in which the endogenous immunoglobulin heavy chain variable region gene locus has been replaced in situ, in part, with the orthologous human gene locus.