In vitro compartmentalization (IVC) combined with microfluidics systems enable the miniaturization of the reaction volume to microdoplets. Single bacterial cells or in vitro transcription/translation systems can be compartmentalised into these droplets, each expressing a different enzyme variant. The compartment maintains a linkage between the overexpressed gene, the enzyme, and the resulting reaction product, combining genotype and phenotype. Using e.g. a microfluidic fluorescence-activated droplet sorting now enables the screening of more than 10e9 samples within a few days, combining advantages of microtitier-plate screening and traditional fluorescence-activated cell sorting (FACS).