A cell comprising an antibody kappa light chain locus (first locus), wherein the locus comprises (in 5' to 3' direction) (a) a light chain EiK intronic enhancer; (b) a rearranged antibody variable region encoding a rearranged antibody V domain; and (c) an antibody light chain constant region encoding a light chain C domain; wherein the locus is operable to express an antibody chain comprising (in N- to C-terminal direction) said rearranged V domain and said C domain, wherein the cell genome comprises a second antibody locus, wherein the second locus is an unrearranged antibody heavy chain locus comprising (in 5' to 3' direction) (a) VH gene segments; (b) one or more DH gene segments; (c) one or more JH gene segments; and (d) a heavy chain constant region encoding one or more CH domains; wherein the heavy chain locus is operable to express a plurality of heavy chains; and wherein said heavy chain locus comprises (i) all human VH gene segments of the group consisting of IGHV1-3, IGHV1-8, IGHV1-18, IGHV1-46, IGHV3-7, IGHV3-9, IGHV3-11, IGHV3-15, IGHV3-20, IGHV3-21, IGHV3-23, IGHV3-30, IGHV3-33, IGHV3-48, IGHV3-53, IGHV4-4, IGHV4-31, IGHV4-34, IGHV4-39, IGHV4-59, IGHV4-61, IGHV5-51, IGHV6-1 and IGHV7-4-1; or (ii) at least 5 human VH gene segments of the group consisting of IGHV1-3*01, IGHV1-8*01, IGHV1-18*01, IGHV1-46*03, IGHV3-7*01, IGHV3-9*01, IGHV3-11*01, IGHV3-15*01, IGHV3-20*d01, IGHV3-21*03, IGHV3-23*04, IGHV3-30*18, IGHV3-33*01, IGHV3-48*02, IGHV3-53*01, IGHV4-4*02, IGHV4-31*03, IGHV4-34*01, IGHV4-39*01, IGHV4-59*01, IGHV4-61*01, IGHV5-51*01, IGHV6-1*01 and IGHV7-4-1*01; (a) a light chain EiK intronic enhancer; (b) a rearranged antibody variable region encoding a rearranged antibody V domain; and (c) an antibody light chain constant region encoding a light chain C domain; wherein the locus is operable to express an antibody chain comprising (in N- to C-terminal direction) said rearranged V domain and said C domain, wherein the cell genome comprises a second antibody locus, wherein the second locus is an unrearranged antibody heavy chain locus comprising (in 5' to 3' direction) (a) VH gene segments; (b) one or more DH gene segments; (c) one or more JH gene segments; and (d) a heavy chain constant region encoding one or more CH domains; (a) VH gene segments; (b) one or more DH gene segments; (c) one or more JH gene segments; and (d) a heavy chain constant region encoding one or more CH domains; wherein the heavy chain locus is operable to express a plurality of heavy chains; and wherein said heavy chain locus comprises (i) all human VH gene segments of the group consisting of IGHV1-3, IGHV1-8, IGHV1-18, IGHV1-46, IGHV3-7, IGHV3-9, IGHV3-11, IGHV3-15, IGHV3-20, IGHV3-21, IGHV3-23, IGHV3-30, IGHV3-33, IGHV3-48, IGHV3-53, IGHV4-4, IGHV4-31, IGHV4-34, IGHV4-39, IGHV4-59, IGHV4-61, IGHV5-51, IGHV6-1 and IGHV7-4-1; or (ii) at least 5 human VH gene segments of the group consisting of IGHV1-3*01, IGHV1-8*01, IGHV1-18*01, IGHV1-46*03, IGHV3-7*01, IGHV3-9*01, IGHV3-11*01, IGHV3-15*01, IGHV3-20*d01, IGHV3-21*03, IGHV3-23*04, IGHV3-30*18, IGHV3-33*01, IGHV3-48*02, IGHV3-53*01, IGHV4-4*02, IGHV4-31*03, IGHV4-34*01, IGHV4-39*01, IGHV4-59*01, IGHV4-61*01, IGHV5-51*01, IGHV6-1*01 and IGHV7-4-1*01; (i) all human VH gene segments of the group consisting of IGHV1-3, IGHV1-8, IGHV1-18, IGHV1-46, IGHV3-7, IGHV3-9, IGHV3-11, IGHV3-15, IGHV3-20, IGHV3-21, IGHV3-23, IGHV3-30, IGHV3-33, IGHV3-48, IGHV3-53, IGHV4-4, IGHV4-31, IGHV4-34, IGHV4-39, IGHV4-59, IGHV4-61, IGHV5-51, IGHV6-1 and IGHV7-4-1; or (ii) at least 5 human VH gene segments of the group consisting of IGHV1-3*01, IGHV1-8*01, IGHV1-18*01, IGHV1-46*03, IGHV3-7*01, IGHV3-9*01, IGHV3-11*01, IGHV3-15*01, IGHV3-20*d01, IGHV3-21*03, IGHV3-23*04, IGHV3-30*18, IGHV3-33*01, IGHV3-48*02, IGHV3-53*01, IGHV4-4*02, IGHV4-31*03, IGHV4-34*01, IGHV4-39*01, IGHV4-59*01, IGHV4-61*01, IGHV5-51*01, IGHV6-1*01 and IGHV7-4-1*01;