Genetic selections using double emulsion compartments and a fluorescence-activated cell sorter (FACS). The water-in-oil emulsion typically contains only one gene variant per microdroplet, enabling noncovalent coupling of genotype to phenotype. Each gene (purple) is transcribed into an mRNA (blue) that is translated into an enzyme (red). Genes encoding enzymes that convert substrate into fluorescent product are selected by FACS through multiple cycles.