Library of a collection of cells

The present invention relates to combinatorial gene expression libraries and methods for making these. The library comprises a collection of individual cells, the cells being denoted cell1,cell2 . . . , cell, wherein i>2, each cell comprising at least one concatemer of individual oligonucleotide cassettes, each concatemer comprising a nucleotide sequence of the following formula: (rs2-SP-PR-X-TR-SP-rs1) n wherein rs1 and rs2 together denote a restriction site, SP denotes a spacer of at least two bases, X denotes an expressible nucleotide sequence, PR denotes a promoter, capable of regulating the expression of X in the cell, TR denotes a terminator, and n 2, and wherein at least one concatemer of cell1 is different from a concatemer cell2. Such libraries are useful in discovery of novel and/or enhanced metabolic pathways leading to the production of novel compounds for e.g. drug discovery and/or to the production of known compounds in novel quantities or in novel compartments of the cells. The expression libraries in particular are composed of host cells capable of co-ordinated and controllable expression of large numbers of heterologous gene in the host cells co-ordinated and controllable expression of large numbers of heterologous gene i host cells.