CRISPR/cas9 endonuclease-mediated genome editing is used to replace the mouse Ighg1 locus hinge (exon 2), CH2 (exon 3) and CH3 (exon 4) regions with a 902 bp human hinge (exon 2), CH2 (exon 3) and CH3 (exon 4) sequences while preserving the splice junctions. (J:296090)