A method for preparing a recombinant host cell library expressing a library of bispecific antibodies comprising: a) providing a targeted integration host cell comprising an exogenous nucleotide sequence integrated at a site within a locus of the genome of the host cell, wherein the exogenous nucleotide sequence comprises a first and a second recombination recognition sequence flanking at least one first selection marker, and a third recombination recognition sequence located between the first and the second recombination recognition sequence, and all the recombination recognition sequences are different; b) introducing into the cell provided in a) a library of first vectors each comprising two recombination recognition sequences matching the first and the third recombination recognition sequence on the integrated exogenous nucleotide sequence and flanking two exogenous nucleotide sequences and at least one second selection marker, and a library of second vectors each comprising two recombination recognition sequences matching the second and the third recombination recognition sequence on the integrated exogenous nucleotide sequence and flanking at least two further exogenous nucleotide sequences; wherein one of the four exogenous nucleotide sequences encode a first light chain, one a second light chain, one a first heavy chain and one a second heavy chain of the bispecific antibody; c) introducing i) either simultaneous with the libraries of first and second vectors of b); or ii) sequentially thereafter one or more recombinases, wherein the one or more recombinases recognize the recombination recognition sequences of the first and second vectors; wherein the one or more recombinases recognize the recombination recognition sequences and perform two recombinase mediated cassette exchanges; and d) selecting for recombinant host cells expressing the second selection marker and secreting a bispecific antibody, thereby preparing a recombinant host cell library expressing a library of bispecific antibodies. a) providing a targeted integration host cell comprising an exogenous nucleotide sequence integrated at a site within a locus of the genome of the host cell, wherein the exogenous nucleotide sequence comprises a first and a second recombination recognition sequence flanking at least one first selection marker, and a third recombination recognition sequence located between the first and the second recombination recognition sequence, and all the recombination recognition sequences are different; b) introducing into the cell provided in a) a library of first vectors each comprising two recombination recognition sequences matching the first and the third recombination recognition sequence on the integrated exogenous nucleotide sequence and flanking two exogenous nucleotide sequences and at least one second selection marker, and a library of second vectors each comprising two recombination recognition sequences matching the second and the third recombination recognition sequence on the integrated exogenous nucleotide sequence and flanking at least two further exogenous nucleotide sequences; wherein one of the four exogenous nucleotide sequences encode a first light chain, one a second light chain, one a first heavy chain and one a second heavy chain of the bispecific antibody; c) introducing i) either simultaneous with the libraries of first and second vectors of b); or ii) sequentially thereafter one or more recombinases, wherein the one or more recombinases recognize the recombination recognition sequences of the first and second vectors; wherein the one or more recombinases recognize the recombination recognition sequences and perform two recombinase mediated cassette exchanges; and d) selecting for recombinant host cells expressing the second selection marker and secreting a bispecific antibody,