According to a first aspect of the invention, a method for the generation of a cell line is provided, comprising the steps of (a) providing a plurality of mammalian B cells, wherein each of the plurality of B cells comprises a transgenic genomic DNA sequence encoding a marker protein inserted into an endogenous immunoglobulin locus comprised in said B cell, and wherein the transgenic genomic DNA sequence is amenable to cleavage by a site directed nuclease, particularly Cas9: (b) replacing the transgenic genomic DNA sequence encoding a marker protein with a second transgenic DNA sequence encoding a protein of interest; (c) sorting B cells based on the presence or absence of the marker protein; and (d) collecting B cells in which the marker protein is absent.

Strategies, systems, methods, reagents, and kits for phage-assisted continuous evolution are provided herein. These include strategies, systems, methods, reagents, and kits allowing for stringency modulation to evolve weakly active or inactive biomolecule variants, negative selection of undesired properties, and/or positive selection of desired properties.

Strategies, systems, methods, reagents, and kits for phage-assisted continuous evolution are provided herein. These include strategies, systems, methods, reagents, and kits allowing for stringency modulation to evolve weakly active or inactive biomolecule variants, negative selection of undesired properties, and/or positive selection of desired properties.