The present invention relates to a method for producing a panel of cells (i.e. a cell library) expressing various different mutant variants of a protein of interest, wherein only one of said mutant variants is expressed per cell from a single gene copy. The present invention also relates to a method or cell library for identifying a mutant variant of a protein of interest having a different or modified biological activity as compared to the corresponding wild-type protein of interest. According to the present invention the identified mutant variant of a protein of interest may be applied for white biotechnology.