The invention provides for a method of producing a mutant somatic human cell line of cells comprising a genomic mutation of interest (MOI) at a predefined genomic site of interest (GOI) in close proximity to a genomic target site, which comprises: a) providing a guide RNA (gRNA) comprising a tracrRNA in conjunction with crRNA including an oligonucleotide sequence that hybridizes with the target site; b) providing an RNA-guided endonuclease which catalyzes the DNA break at the target site upon hybridizing with the gRNA; c) introducing the gRNA into the cells in the presence of the endonuclease to obtain a repertoire of cells comprising a variety of genomic mutations at the target site; d) selecting a cell from said repertoire which comprises a MOI; wherein the cell is haploid for the genomic locus of the target site; and e) expanding the cell to obtain the mutant cell line. The invention further provides for a mutant human somatic cell line obtainable by such method; and libraries of mutant human somatic cell lines of isogenic cells with a variety of genomic mutations at different predefined genomic target sites.