Indexed library of cells containing genomic modifications and methods of making and utilizing the same

A library of cultured eucaryotic cells comprising at least two subpopulations of cells, wherein the at least two subpopulations of cells comprises

• (a) a first subpopulation of cells, wherein cells of the first subpopulation have a first vector integrated nonspecifically into their genomes, wherein the first vector mediates the splicing of a foreign exon internal to a cellular transcript, and wherein the first vector comprises:
• (i) a foreign exon,
• (ii) a splice acceptor site operatively positioned 5′ to said foreign exon, and
• (iii) a splice donor site operatively positioned 3′ to said foreign exon, wherein the first vector does not comprise a promoter operatively positioned 5′ to said foreign exon;
• and
• (b) a second subpopulation of cells, wherein cells of the second subpopulation have a second vector integrated nonspecifically into their genomes, wherein the second vector mediates the splicing of a foreign exon 5′ to an exon of a cellular transcript, and wherein the second vector comprises:
• (i) a foreign exon,
• (ii) a promoter operatively positioned 5′ to said foreign exon,
• (iii) a splice donor site operatively positioned 3′ to said foreign exon;
• and
• (iv) a mutagenic foreign polynucleotide sequence located upstream from said promoter.